Low-temperature Photoluminescence Research Articles

Isostructural Single- And Dual-Lanthanide Metal-Organic Frameworks Based On Substituent-Group-Modifying Tetracarboxylate Ligands for Ratiometric Temperature Sensing.

In this study, two substituent-group-modifying tetracarboxylate ligands, 2',5'-dimethoxy-[1,1':4',1″-terphenyl]-3,3″,5,5″-tetracarboxylic acid (H4TPTC-2OMe) and 2',5'-dimethyl-[1,1':4',1″-terphenyl]-3,3″,5,5″-tetracarboxylic acid (H4TPTC-2Me), with similar geometries were used as the organic linkers to construct isostructural lanthanide metal-organic frameworks (LnMOFs). The as-prepared LnTPTC-2OMe and LnTPTC-2Me were structurally elucidated by means of single-crystal and powder X-ray diffraction in addition to thermogravimetric analysis and were assessed as luminescence ratiometric thermometers by obtaining the temperature dependence of the luminescence behaviors. We found that both the single lanthanide EuTPTC-2OMe and the dual lanthanide Eu0.05Tb0.95TPTC-2Me exhibited a distinct S-type luminescence response to temperatures in the range from 313 to 473 K, and their ratiometric parameters can be understood on the basis of the classic Mott-Seitz model. Energy transfers from the ligand to Tb3+ (or Eu3+) and from Tb3+ to Eu3+ in these two systems were investigated theoretically as well as with low-temperature (77 K) time-resolved photoluminescence spectroscopy, quantum yield, and lifetime analysis. Therefore, these two materials possess a good relative sensitivity, a small temperature uncertainty, and a favorable spectral repeatability in addition to a remarkable emission color change, enhancing their potential use for temperature measurement and in situ monitoring in microelectronics.

Broad band photoluminescence of g-C3N4/ZnO/ZnS composite towards white light source

We have found one step synthesized g–C3N4/ZnO/ZnS composite to be promising for engineering of white-light sources. The composite was fabricated by simultaneous pyrolytic decomposition of thiourea and zinc acetate with a consequent insitu polymerization of the products at 550–625 °C and characterized by SEM, XRD, EDX and low temperature PL spectroscopy. The composite synthesized at 550 °C demonstrates blue PL with a peak at 2.74 eV while higher temperatures of the synthesis result in a broad spectra with a maximum at 2.22 eV and comprise of the bands of g–C3N4 and mixed ZnO/ZnS as they have been resolved by low temperature PL. The proper selection of the synthesis temperature allows an extension of light emitting spectra of g–C3N4/ZnO/ZnS composites from blue to white light range. The results obtained show ternary systems containing g-C3N4 and wide band-gap semiconductors are suitable for white-light emitting and optoelectronic devices.

Origin of blue luminescence in Mg-doped GaN

In this work, dual acceptor-bound exciton peaks are observed by low-temperature photoluminescence. The peaks correspond to the dual Mg-related acceptor levels in GaN based on the Haynes rule. By calibrating the energy-level structure, a mechanism for the origin of blue luminescence (BL) in Mg-doped GaN is proposed. The BL band is separated by thermal treatment at different temperatures, confirming the rationality of the dual-factor origin of the BL band. As the annealing temperature increases, the PL spectrum and the p-type conductivity of Mg-doped GaN also change. The experimental results indicate that there is not necessarily a relationship between the BL band and p-type conductivity in GaN grown by metalorganic chemical vapor deposition.

Exciton radiative lifetime in a monoatomic carbon chain

Linear carbon-based materials such as polyyne and cumulene oligomers provide a versatile platform for nano-physics and engineering. Direct gap quasi-1D polyyne structures are promising for the observation of strong and unusual excitonic effects arising due to the two-dimensional quantum confinement. Recently, we reported on the observation of sharp exciton peaks in low temperature photoluminescence spectra of polyyne chains (Kutrovskaya S et al 2020 Nano Lett. 20 6502–9). Here, we analyze the time-resolved optical response of this system. We extend the non-local dielectric response theory to predict the exciton radiative lifetime dependence on the band-gap value and on the length of the chain. A good agreement between the experiment and the theory is achieved.

Photoluminescence excitation spectroscopy for structural and electronic characterization of resonant tunneling diodes for THz applications

Photoluminescence excitation spectroscopy (PLE) and high-resolution x-ray diffraction (HR-XRD) are used to characterize the structural and electronic properties of high current density InGaAs/AlAs/InP resonant tunneling diode wafer structures. The non-destructive assessment of these structures is challenging, with several unknowns: well and barrier thickness, the well indium molar fraction, and band-offsets, which are a function of strain, material, growth sequence, etc. The low temperature PL spectra are deconvoluted through simulation and are shown to include contributions from type I (e1–hh1) and type II (conduction band–hh1) transitions that are broadened due to interface fluctuations on a range of length scales. PLE data are obtained by a careful choice of the detection wavelength, allowing the identification of the e2hh2 transition that is critical in determining the band-offsets. An agreement between the HR-XRD data, the PL, and the PLE data is only obtained for a given conduction band offset of 58.8%. This scheme, combining HR-XRD, PL, and PLE, consequently provides crucial electronic and structural information non-destructively.

Structural and optical properties of self-assembled AlN nanowires grown on SiO2/Si substrates by molecular beam epitaxy

Self-assembled AlN nanowires (NWs) are grown by plasma-assisted molecular beam epitaxy (PAMBE) on SiO2/Si (111) substrates. Using a combination of in situ reflective high energy electron diffraction and ex situ x-ray diffraction (XRD), we show that the NWs grow nearly strain-free, preferentially perpendicular to the amorphous SiO2 interlayer and without epitaxial relationship to Si(111) substrate, as expected. Scanning electron microscopy investigation reveals significant NWs coalescence, which results in their progressively increasing diameter and formation of columnar structures with non-hexagonal cross-section. Making use of scanning transmission electron microscopy (STEM), the NWs initial diameters are found in the 20–30 nm range. In addition, the formation of a thin (≈30 nm) polycrystalline AlN layer is observed on the substrate surface. Regarding the structural quality of the AlN NWs, STEM measurements reveal the formation of extended columnar regions, which grow with a virtually perfect metal-polarity wurtzite arrangement and with extended defects only sporadically observed. Combination of STEM and electron energy loss spectroscopy reveals the formation of continuous aluminum oxide (1–2 nm) on the NW surface. Low temperature photoluminescence measurements reveal a single near-band-edge (NBE) emission peak, positioned at 6.03 eV (at 2 K), a value consistent with nearly zero NW strain evidenced by XRD and in agreement with the values obtained on AlN bulk layers synthesized by other growth techniques. The significant full-width-at-half-maximum of NBE emission, found at ≈20 meV (at 2 K), suggests that free and bound excitons are mixed together within this single emission band. Finally, the optical properties of the hereby reported AlN NWs grown by PAMBE are comprehensively compared to optical properties of bulk, epitaxial and/or columnar AlN grown by various techniques such as: physical vapor transport, metal organic vapor phase epitaxy, metal organic chemical vapor deposition and molecular beam epitaxy.

Microstructural and optical properties of stress-free GaN films on graphene prepared by PECVD

The van der Waals epitaxy (vdWE) of three-dimensional (3D) semiconductors on 2D materials has broad prospects for integrating the unique advantages of both. In this study, multilayer graphene (MLG) was synthesized by plasma enhanced chemical vapor deposition (PECVD) on sapphire and then flat GaN thin films were obtained through metal organic chemical vapor deposition (MOCVD) assisted by the sputtering of an AlN buffer layer. Transmission electron microscopy (TEM) showed the existence of low-angle grain boundaries and mainly mixed dislocations at the interface of the epitaxial GaN. As the thickness of the GaN film increased, the number of low-angle grain boundaries dropped by an order of magnitude over the initial 1–3 μm. Spectral lines typical of a stress-free GaN film were seen in the low-temperature photoluminescence (PL) measurements, and the TEM analysis corresponded well with the luminescent peaks. These detailed characterizations offer an avenue for a better understanding of the growth behavior of GaN films grown on graphene and furthermore, for obtaining high-quality GaN.

Low temperature photoluminescence and solvatochromic studies of organic hybrid 4-methoxybenzylammonium chloride (4-MBACl)

We report temperature dependent photoluminescence (PL) and solvatochromic studies of 4-methoxybenzylammonium chloride (4-MBACl). The single crystals of 4-MBACl has been grown using slow solvent evaporation method under ambient conditions and have been confirmed using X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopic studies. Thermal stability of the material has been analyzed by thermogravimetric (TG) and differential thermal analyzes (DTA). 4-MBACl crystal is photoluminescent with emission wavelength maximum at 300 nm. Exciton-phonon coupling in 4-MBACl is realized from temperature dependent PL studies. Changes in full width at half maximum (FWHM) of PL emission peak, intensity maximum (Imax) and wavelength maximum (λmax) have been analyzed systematically between temperatures 290–10 K in order to understand the photophysical phenomenon behind the emission. Optical band gap (Eg) of 4-MBACl has been varied using solvent polarity, proticity and solution concentration. Interactions like OH---O, OH---π and π-π stacking are explained to reason the notable shifts.

Atomic-like emissions in UV region from hexagonal boron nitride crystal powders

Hexagonal boron nitride (h-BN) crystal powders were characterized for structural and optical properties. Deep UV photoluminescence (PL) spectroscopy was employed for the optical characterization. We studied the effect of annealing on the PL emissions from the h-BN crystal powders. We observed sharp atomic-like PL emission lines in the UV spectral region at 4.10 and 4.12 eV along with their longitudinal optical phonon replicas from the samples annealed in ambient air around 700 °C for 1 h which were not present in the unannealed samples. Our study suggests that the point defects related to the atomic-like emissions are generated during the annealing process of the h-BN. Based on the theoretical calculation results, the atomic-like emissions in the UV region have been assigned to a carbon dimer defect. The annealed h-BN powder samples also exhibited pronounced phonon assisted band-edge emission peaks at 5.78 and 5.89 eV, and enhanced emission intensity of the major peak at 5.49 eV in the low temperature (8 K) PL spectrum.

Study on the stress and mechanism of self-separated GaN grown by Na-flux method

A 2 inch free-standing c-plane GaN wafer was fabricated through in situ self-separation using HVPE-seed crystal etching back (HCEB) by intentionally adjusting the nitrogen pressure in the Na-flux growth process of GaN. First, adjust the nitrogen pressure in the reactor to a lower level to facilitate HCEB to form a large number of voids at the interface between the c-plane HVPE seed and the c-plane Na-flux GaN. After regrowth of approximately 340 μm thick Na-flux GaN, self-separation was achieved during the cooling process. The free-standing GaN wafer was almost stress-free as a result of strain relief by the in situ self-separation process, which was confirmed by room-temperature Raman and low-temperature photoluminescence measurements. It is supposed that the HCEB process can be applied to fabricate high-quality free-standing GaN wafers in the future.

Low-temperature photoluminescence of C60 single crystals intercaled with nitrogen molecules in the wide range of temperatures

The optical properties of C60 single crystals, intercalated with nitrogen molecules, were studied by the spectral-luminescence method at a temperature T = 30 K and excitation by the He–Ne laser (Eexc = 1.96 eV). Intercalation was carried out at a pressure of 30 atm in a temperature range of 200–550 °C. It was found that at sorption temperatures up to 400 °C, the bands of the low-temperature luminescence spectrum of the C60–N2 solutions are broadened without significant energy shift. As a rule, this situation is typical for the case of an increased contribution of the luminescence of “deep X-traps.” The concentration of such exciton emission centers is determined by the degree of occupation of the octahedral cavities of the fullerite fcc lattice by intercalated molecules. This indicates the formation of an equilibrium C60–N2 interstitial solution as a result of physisorption. At sorption temperatures above 400 °C, a significant shift of the luminescence spectrum towards low energies with a considerable inhomogeneous broadening of spectral bands was found for the first time. The shift and significant nonuniform broadening of the luminescence bands are explained by the emergence of a chemical interaction (chemisorption) of nitrogen with fullerene molecules, with the formation of a new nitrogen-containing substance in this case.

Formation of InAs/GaP Quantum-Well Heterostructures on Silicon Substrates by Molecular-Beam Epitaxy

The possibility of forming a strained pseudomorphous quantum well (QW) consisting of a InxGa1 – xAsyP1 – y quaternary alloy upon the deposition of InAs onto the surface of an epitaxial GaP/Si layer with a developed relief is demonstrated. The QW is studied by means of transmission electron microscopy and steady-state photoluminescence spectroscopy. The formation of two QW segments different in width and composition of the InxGa1 – xAsyP1 – y alloy is observed; in this case, an increase in the QW width is accompanied by a decrease in the content of In and As atoms. The lateral dimensions of the QW segments are no smaller than 20 nm. The QW segments correspond to two different low-temperature photoluminescence bands. The experimentally observed phenomena are interpreted on the assumption of transformation of the surface under the action of elastic strains during heteroepitaxy of InAs on the terraced GaP surface.

Investigations of Electron-Electron and Interlayer Electron-Phonon Coupling in van der Waals hBN/WSe2/hBN Heterostructures by Photoluminescence Excitation Experiments.

Monolayers of transition metal dichalcogenides (TMDs) with their unique physical properties are very promising for future applications in novel electronic devices. In TMDs monolayers, strong and opposite spin splittings of the energy gaps at the K points allow for exciting carriers with various combinations of valley and spin indices using circularly polarized light, which can further be used in spintronics and valleytronics. The physical properties of van der Waals heterostructures composed of TMDs monolayers and hexagonal boron nitride (hBN) layers significantly depend on different kinds of interactions. Here, we report on observing both a strong increase in the emission intensity as well as a preservation of the helicity of the excitation light in the emission from hBN/WSe2/hBN heterostructures related to interlayer electron-phonon coupling. In combined low-temperature (T = 7 K) reflectivity contrast and photoluminescence excitation experiments, we find that the increase in the emission intensity is attributed to a double resonance, where the laser excitation and the combined Raman mode A′1 (WSe2) + ZO (hBN) are in resonance with the excited (2s) and ground (1s) states of the A exciton in a WSe2 monolayer. In reference to the 2s state, our interpretation is in contrast with previous reports, in which this state has been attributed to the hybrid exciton state existing only in the hBN-encapsulated WSe2 monolayer. Moreover, we observe that the electron-phonon coupling also enhances the helicity preservation of the exciting light in the emission of all observed excitonic complexes. The highest helicity preservation of more than 60% is obtained in the emission of the neutral biexciton and negatively charged exciton (trion) in its triplet state. Additionally, to the best of our knowledge, the strongly intensified emission of the neutral biexciton XX0 at double resonance condition is observed for the first time.

Etching-induced damage in heavily Mg-doped p-type GaN and its suppression by low-bias-power inductively coupled plasma-reactive ion etching

Inductively coupled plasma–reactive ion etching (ICP–RIE)-induced damage in heavily Mg-doped p-type GaN ([Mg] = 2 × 1019 cm−3) was investigated by low-temperature photoluminescence (PL) and depth-resolved cathodoluminescence (CL) spectroscopy. From PL measurements, we found broad yellow luminescence (YL) with a maximum at around 2.2–2.3 eV, whose origin was considered to be isolated nitrogen vacancies (V N), only in etched samples. The depth-resolved CL spectroscopy revealed that the etching-induced YL was distributed up to the electron-beam penetration depth of around 200 nm at a high ICP–RIE bias power (P bias). Low-bias-power (low-P bias) ICP–RIE suppressed the YL and its depth distribution to levels similar to those of an unetched sample, and a current–voltage characteristic comparable to that of an unetched sample was obtained for a sample etched with P bias of 2.5 W.

N-type GaN surface etched green light-emitting diode to reduce non-radiative recombination centers

In this study, we attempt to identify the presence of surface defects (SDs) at an n-type GaN surface after high-temperature growth and gain insight into their intrinsic features. To this end, first, we carefully investigate n-type GaN samples with different surface etching depths. Low-temperature photoluminescence (PL) spectra reveal that SDs are most likely nitrogen vacancies (VN) and/or VN-related point defects intensively distributed within ∼100 nm from the n-type GaN surface after a high-temperature growth. We investigate the effect of SDs on the internal quantum efficiency (IQE) of green light-emitting diodes (LEDs) by preparing GaInN-based green LEDs employing a surface-etched n-type GaN, which exhibits a prominent enhancement of the PL efficiency with an increase in the etching depth. This effect is attributable to the reduced non-radiative recombination centers in multiple-quantum-well active regions because the SDs near the n-type GaN surface are removed by etching. We discuss strategies of in situ engineering on SDs to further improve the IQE in GaInN-based green LEDs on the basis of the results presented in this study.

Hybrid strain-coupled multilayer SK and SML InAs/GaAs quantum dot heterostructure: Enabling higher absorptivity and strain minimization for enhanced optical and structural characteristics

The size and shape of self-assembled quantum dots (QDs) in the top layer of a strain-coupled multilayer Stranski-Krastanov (SK) type QD heterostructure is decided by the effect of strain from the bottom QD layers along with the compressive strain through InAs/GaAs interface. A novel approach is used in this article to grow the heterostructures such that QDs from one layer to the other are heterogeneously coupled resulting in reduced cumulative strain, which helps in formation of defect free and uniform dots even for 10 strain-coupled SK QD layers. Heterostructures with strain-coupled single- ( × 1), bi- ( × 2), tri- ( × 3), penta- ( × 5), hepta- ( × 7) and deca- ( × 10) layer SK QDs are grown employing a growth mechanism on six stack sub-monolayer (SML) type QDs, considering the individual advantages of SK and SML QDs. Full width half maximum (FWHM) calculated from the low temperature (19 K) photoluminescence (PL) are 50, 52, 54, 55, 56 and 50 nm for single, bi, tri, penta, hepta and deca-layer SK QD stack respectively, which shows uniform dot size distribution up to deca-layer SK QD structure because of the adapted growth mechanism. The activation energies calculated from temperature dependent PL are 244, 281, 299, 275, 288 and 397 meV for single, bi, tri, penta, hepta and deca-layers respectively, which aids that the strain-coupled deca-layer SK QD heterostructure has the highest activation energy compared to other strain-coupled multilayer SK QD structures. These properties make the deca-layer SK QD structure as the optimum device for quantum dot infrared photodetectors (QDIPs), which could operate at higher temperatures with improved performance parameters. High resolution X-ray diffraction (HRXRD) shows a defect free heterostructure with minimized hydrostatic strain and homogeneous dot size distribution for the strain-coupled deca-layer SK QD structure. Thus, the adapted growth mechanism along with heterogeneous coupling of SK and SML QDs facilitates higher absorption efficiency, increased carrier lifetime and better quantum confinement, making it a promising design to be used in intermediate band solar cells (IBSC) and QDIPs, where multilayer QDs are needed so as to improve the device performance.

Heteroepitaxial growth of wide bandgap cuprous iodide films exhibiting clear free-exciton emission

Cuprous iodide (CuI) is an emerging wide-bandgap semiconductor of superior optical and transport properties. In particular, CuI shows high stability and large oscillator strength of free excitons that are of great advantage for optoelectronic applications. However, thin films of CuI reported so far have not been genuine single crystals, containing a sizable density of impurity and defect. Here, we demonstrate a dramatic improvement in the quality of CuI films grown by molecular beam epitaxy on a lattice-matched InAs substrate. The film is revealed to be in a single-crystal structure with high lattice coherence and an atomically flat surface. The low-temperature photoluminescence spectra exhibit extremely sharp emission from free excitons and much-suppressed emission from trapped states. The high-quality CuI films realized in the present study will not only facilitate the device application of CuI films but also provide unprecedented functionalities in halide semiconductors at the atomically sharp heterointerfaces.

Room temperature synthesis and characterization of novel lead-free double perovskite nanocrystals with a stable and broadband emission

We demonstrated the synthesis of ultra small and stable Cs3BiBr6 nanocrystals, ∼1.5–3 nm, via a room-temperature antisolvent method. Red-shift of bandgap was observed in low temperature PL measurements with an activation energy of ∼41 meV.

Field-effect-transistor-based ion sensors: ultrasensitive mercury(II) detection via healing MoS2 defects.

The contamination of water with heavy metal ions represents a harsh environmental problem resulting from societal development. Among various hazardous compounds, mercury ions (Hg2+) surely belong to the most poisonous ones. Their accumulation in the human body results in health deterioration, affecting vital organs and eventually leading to chronic diseases, and, in the worst-case scenario, early death. High selectivity and sensitivity for the analyte of choice can be achieved in chemical sensing using suitable active materials capable of interacting at the supramolecular level with the chosen species. Among them, 2D transition metal dichalcogenides (TMDCs) have attracted great attention as sensory materials because of their unique physical and chemical properties, which are highly susceptible to environmental changes. In this work, we have fabricated MoS2-based field-effect transistors (FETs) and exploited them as platforms for Hg2+ sensing, relying on the affinity of heavy metal ions for both point defects in TMDCs and sulphur atoms in the MoS2 lattice. X-ray photoelectron spectroscopy characterization showed both a significant reduction of the defectiveness of MoS2 when exposed to Hg2+ with increasing concentration and a shift in the binding energy of 0.2 eV suggesting p-type doping of the 2D semiconductor. The efficient defect healing has been confirmed also by low-temperature photoluminescence measurements by monitoring the attenuation of defect-related bands after Hg2+ exposure. Transfer characteristics in MoS2 FETs provided further evidence that Hg2+ acts as a p-dopant of MoS2. Interestingly, we observed a strict correlation of doping with the concentration of Hg2+, following a semi-log trend. Hg2+ concentrations as low as 1 pM can be detected, being way below the limits imposed by health regulations. Electrical characterization also revealed that our sensor can be efficiently washed and used multiple times. Moreover, the developed devices displayed a markedly high selectivity for Hg2+ against other metal ions as ruled by soft/soft interaction among chemical systems with appropriate redox potentials, being a generally applicable approach to develop chemical sensing devices combining high sensitivity, selectivity and reversibility, to meet technological needs.

Arsenic Doping Upon the Deposition of CdTe Layers from Dimethylcadmium and Diisopropyltellurium

The incorporation and activation of arsenic from tris(dimethylamino)arsine in CdTe layers grown by metal-organic chemical vapor deposition from dimethylcadmium and diisopropyltellurium on GaAs substrates are investigated. The incorporation of arsenic into CdTe depends on the crystallographic orientation of the layers and increases in the series (111)B → (100) → (310). The arsenic concentration in the CdTe layers is proportional to the tris(dimethylamino)arsine flow rate at a power of 1.4 and increases with decreasing diisopropyltellurium/dimethylcadmium ratio from 1.4 to 0.5. After deposition, the CdTe:As layers have p-type conductivity with an arsenic concentration of 1 × 1017–7 × 1018 cm–3 and a hole concentration of 2.7 × 1014–4.6 × 1015 cm–3, respectively; the fraction of electrically active arsenic does not exceed ~0.3%. After annealing in argon (250–450°C), the highest hole concentration is 1 × 1017 cm–3, and the arsenic activation efficiency is ~4.5%. The ionization energy of arsenic determined from the temperature dependence of the hole concentration is in the range of 98–124 meV. The low-temperature photoluminescence spectra of the layers have an emission peak with an energy of ~1.51 eV, which can be attributed to donor–acceptor recombination, where AsTe is an acceptor with an ionization energy of ~90 meV.