Nonradiative Recombination Channels Research Articles

Temperature- and size-dependent photoluminescence in colloidal CdTe and Cd x Zn1−x Te quantum dots

Semiconductor colloidal quantum dots (QDs) of CdTe and alloyed Cd x Zn1−x Te QDs with N-acetyl-L-cysteine capping ligands are synthesized by a reflux method in aqueous solution. Alloying provides a new degree of freedom to tune the optical and electronic properties of the nanocrystals. The photoluminescence (PL) of Cd x Zn1−x Te QDs is sharper and displays a highly enhanced quantum yield (QY) of 65% relative to the 16% of CdTe QDs. The fluorescence of Cd x Zn1−x Te QDs is observed to be highly stable for over 12 months without degradation, while that of CdTe QDs begins to mildly flocculate around 8 months of storage. To characterise the material structure and composition, UV-Vis absorption spectroscopy, x-ray powder diffraction, transmission electron microscopy, and inductively coupled plasma mass spectrometry measurements are carried out. To understand the fundamental processes that play in the luminescence behaviour, temperature- and size-dependent PL spectra are investigated in the range 80–300 K. The Varshni and O’Donnell equations fit well on the PL peak emission energies and the Huang–Rhys parameter indicates the strengthening of exciton–phonon coupling in the nanocrystals upon alloying and with decreasing nanocrystal sizes. PL linewidth analysis reveals that the inhomogeneous broadening is considerably reduced in Cd x Zn1−x Te QDs relative to CdTe. Moreover, the quantum confinement effect of the nanocrystals leads to an increase in exciton–acoustic phonon interactions with the coefficients ranging between 26.9 and 95.6 µeV K−1 compared to the bulk CdTe value of 0.72 µeV K−1. Exciton–longitudinal optical phonon interactions are made stronger by the ZnTe alloying with the coefficients lying in the range between 24.8 and 41.7 meV and also with the effect of increasing crystal size. An Arrhenius plot of PL integrated area is used to calculate the thermal activation energy value E a of the non-radiative recombination channel, which is 132 meV for CdTe QDs and a higher value of 185 meV for Cd x Zn1−x Te QDs. This is consistent with the observed QY enhancement in Cd x Zn1−x Te QDs as a higher E a value indicates reduced generation of non-radiative recombination centres and a decrease in defect densities upon alloying. Cd x Zn1−x Te QDs with enhanced fluorescence properties serve both as a medium for studying fundamental effects of alloying and its properties, and for practical applications such as biomedical labelling and optoelectronics.

Luminescence Quenching via Deep Defect States: A Recombination Pathway via Oxygen Vacancies in Ce-Doped YAG

Luminescence quenching via non-radiative recombination channels limits the efficiency of optical materials such as phosphors and scintillators and therefore has implications for conversion efficiency and device lifetimes. In materials such as Ce-doped yttrium aluminum garnet (YAG:Ce), quenching shows a strong dependence on both temperature and activator concentration, limiting the ability to fabricate high-intensity white-light emitting diodes with high operating temperatures. Here, we reveal by means of first-principles calculations an efficient recombination mechanism in YAG:Ce that involves oxygen vacancies and gives rise to thermally activated concentration quenching. We demonstrate that the key requirements for this mechanism to be active are localized states with strong electron-phonon coupling. These conditions are commonly found for intrinsic defects such as anion vacancies in wide band-gap materials. The present findings are therefore relevant to a broad class of optical materials and shine light on thermal quenching mechanisms in general.

Nonradiative recombination channel of dark excitons in colloidal CdSe nanoplatelets

Nonradiative recombination channel of spin-forbidden dark excitons in colloidal 4 ML thick CdSe nanoplatelets synthesized in argon and ambient atmosphere is revealed by theoretical analysis of time-resolved photoluminescence. The nonradiative recombination lifetime τnr ≈ 11 ns is found to be independent on nanoplatelets synthesis atmosphere. It is shown that the presence of the nonradiative recombination channel affects spin polarization of dark excitons and results in the time dependence of the circular polarization degree of photoluminescence in the external magnetic field.

(Invited) Engineering SiGeSn Semiconductors for MIR and THz Opto-electronic Devices

Sn-containing group IV semiconductors (Si)GeSn represent a versatile platform to implement a variety of Si-compatible photonic, optoelectronic, and photovoltaic devices operating from SWIR to MIR wavelengths. This class of semiconductors provides two degrees of freedom, strain and composition, to tailor the band structure and lattice parameter thus enabling a variety of heterostructures and low-dimensional systems on a Si substrate.[1]In this presentation, the recent progress in the epitaxial growth and opto-electronic properties of metastable (Si)GeSn semiconductors with Sn contents above 15 at.% will be discussed. The growth of the (Si)GeSn multi-layer heterostructure is performed in a chemical vapor deposition (CVD) reactor on a Si wafer. By reducing the growth temperature, the Sn content in the alloy is increased, while preserving a high degree of crystal purity for the heterostructure in the topmost Sn-rich layer. Atom probe tomography (APT) measurements will be discussed to address the abruptness of the interfaces and the compositional profile across the GeSn multi-layer heterostructure.[2-4]By tailoring the strain relaxation in Ge0.83Sn0.17 room-temperature photoluminescence (PL) emission wavelength above 4.0 μm upon is achieved.[2,5] The absence of defect- and impurity-related emission and limited carrier losses into non-radiative recombination channels will be addressed using temperature-dependent PL measurements. Direct band gap absorption will be shown using transmission measurements performed at room-temperature, with energies closely matching the PL data. These observations will be discussed in the light of photocurrent measurements on GeSn photodetectors operating up to ~4.6 μm at room-temperature. The integration of p-i-n heterostructures in GeSn photodetectors will be investigated by correlating structural and opto-electronics properties of the fabricated devices.In addition, strategies to further extend the operational wavelength range of (Si)GeSn devices toward THz wavelengths will be discussed.[1] S. Wirths, et al., Prog. Cryst. Growth Charact. Mater. 62, 1 (2016)[2] S. Assali, et al., Appl. Phys. Lett. 112, 251903 (2018).[3] S. Assali, et al., J. Appl. Phys. 125, 025304 (2019).[4] S. Assali, et al., Appl. Phys. Lett. 114, 251907 (2019)[5] S. Assali, et al., arxiv:2004.13858

Charge‐Carrier Trapping and Radiative Recombination in Metal Halide Perovskite Semiconductors

AbstractTrap‐related charge‐carrier recombination fundamentally limits the performance of perovskite solar cells and other optoelectronic devices. While improved fabrication and passivation techniques have reduced trap densities, the properties of trap states and their impact on the charge‐carrier dynamics in metal‐halide perovskites are still under debate. Here, a unified model is presented of the radiative and nonradiative recombination channels in a mixed formamidinium‐cesium lead iodide perovskite, including charge‐carrier trapping, de‐trapping and accumulation, as well as higher‐order recombination mechanisms. A fast initial photoluminescence (PL) decay component observed after pulsed photogeneration is demonstrated to result from rapid localization of free charge carriers in unoccupied trap states, which may be followed by de‐trapping, or nonradiative recombination with free carriers of opposite charge. Such initial decay components are shown to be highly sensitive to remnant charge carriers that accumulate in traps under pulsed‐laser excitation, with partial trap occupation masking the trap density actually present in the material. Finally, such modelling reveals a change in trap density at the phase transition, and disentangles the radiative and nonradiative charge recombination channels present in FA0.95Cs0.05PbI3, accurately predicting the experimentally recorded PL efficiencies between 50 and 295 K, and demonstrating that bimolecular recombination is a fully radiative process.

NATIVE DEFECTS IN CdZnS QUANTUM DOTS

Defects in quantum (QDs) not only can affect their optical properties, but also create additional channels of radiative and non-radiative recombination. We studied the structural and electronic characteristics of metal (cadmium, zinc) vacancies and interstitial sulfur as the most probable types of defects in Cd1-xZnxS nanocrystals by means the density functional theory (DFT). The total Sensor Electronics and and partial density of electronic states were calculated, the analysis of structural relaxation was carried out. Based on the obtained theoretical results and available experimental data, we concluded that among the all types of defects considered here, only cadmium vacancies can be the centers of radiative recombination in Cd1-xZnxS quantum dots

Two-dimensional electron bound hole photoluminescence in GaAs in perpendicular magnetic fields

The photoluminescence of quasi two-dimensional (2D) electrons in modulation doped AlxGa1 − xAs-GaAs single heterostructure recombining with bound holes was studied at liquid helium temperatures in magnetic fields up to 9 T. Luminescence contributions from the ground and a weakly populated first excited 2D subband were observed as well as bulk-like contributions from the GaAs buffer. The 2D subband and the bulk signals are linked and oscillate with the magnetic field strength. It is not the filling factor that rules but the alignment of the two subband Landau-ladders paired with intersubband scattering that drives an oscillatory behavior of the subband populations. Asymmetric Landau-level transition line shapes indicate a perturbed 2D electron system. We performed a detailed analyses of Landau-level dependent line strengths, positions and half widths and compared the experiment with calculations of subband energies and transition matrix elements. The total electron density is not fixed in our experiment but decreases with magnetic field strength. This might indicate that the 2D system exchanges electrons with a reservoir and/or that the correlation between radiative and nonradiative recombination channels vary strongly with the magnetic field strength.

Highly efficient luminescence from space-confined charge-transfer emitters.

Charge-transfer (CT) complexes, formed by electron transfer from a donor to an acceptor, play a crucial role in organic semiconductors. Excited-state CT complexes, termed exciplexes, harness both singlet and triplet excitons for light emission, and are thus useful for organic light-emitting diodes (OLEDs). However, present exciplex emitters often suffer from low photoluminescence quantum efficiencies (PLQEs), due to limited control over the relative orientation, electronic coupling and non-radiative recombination channels of the donor and acceptor subunits. Here, we use a rigid linker to control the spacing and relative orientation of the donor and acceptor subunits, as demonstrated with a series of intramolecular exciplex emitters based on 10-phenyl-9,10-dihydroacridine and 2,4,6-triphenyl-1,3,5-triazine. Sky-blue OLEDs employing one of these emitters achieve an external quantum efficiency (EQE) of 27.4% at 67 cd m-2 with only minor efficiency roll-off (EQE = 24.4%) at a higher luminous intensity of 1,000 cd m-2. As a control experiment, devices using chemically and structurally related but less rigid emitters reach substantially lower EQEs. These design rules are transferrable to other donor/acceptor combinations, which will allow further tuning of emission colour and other key optoelectronic properties.

Hole capture-coefficient of intrinsic nonradiative recombination centers that commonly exist in bulk, epitaxial, and proton-irradiated ZnO

Wurtzite ZnO and related MgxZn1−xO alloys are attractive semiconductors for the use in radiation-resistant and/or visible-light-transparent transistors and ultraviolet light-emitters. As free-carrier lifetime controls the device performances, the accurate understanding of the carrier capture-coefficients of dominant nonradiative recombination channels is essential. In this paper, the hole capture-coefficient (Cp) at room temperature of major intrinsic nonradiative recombination centers (NRCs) that commonly exist in various low dislocation density n-type epitaxial films and nearly dislocation-free bulk single crystals of ZnO with and without irradiation by an 8 MeV proton beam is determined. A two-component density functional theory calculation with positron annihilation measurement reveals that major vacancy-type defects are divacancies comprised of a Zn-vacancy and an O-vacancy (VZnVO). Because the weak-excitation nonradiative photoluminescence lifetime (τNR) decreases with increasing VZnVO concentration ([VZnVO]), VZnVO are assigned as major NRCs in n-type ZnO. From the relationship between τNR and [VZnVO], the values of Cp and hole capture-cross section of VZnVO are obtained to be 3×10−7cm3s−1 and 2×10−14cm2, respectively, according to the Shockley–Read–Hall approach. These values are an order of magnitude larger than those of 3d transition metals such as Ni or Mn but are comparable to those of major intrinsic NRCs in n-type GaN, i.e., divacancies comprised of a Ga-vacancy and a N-vacancy (VGaVN), being 6×10−7cm3s−1 and 7×10−14cm2, respectively [S. F. Chichibu, A. Uedono, K. Kojima, H. Ikeda, K. Fujito, S. Takashima, M. Edo, K. Ueno, and S. Ishibashi, J. Appl. Phys. 123, 161413 (2018)].

Luminescence Degradation Mechanisms in CdS/ZnSe Colloidal Nanocrystals

The results of the study of one-photon processes of optically induced degradation of photoluminescence intensity of CdS/ZnSe nanocrystals are presented, and mechanisms of this degradation are determined. It is shown that the nanocrystal photodegradation process consists of three components. The first degradation component is caused by Auger recombination activation in photoinduced hole trapping by the trap associated with a surface state; a long-term degradation component can be associated with thermally activated chemical bond breaking between a passivating ligand, i.e., oleic acid, and the nanocrystal surface followed by nonradiative recombination channel activation.

Excitation wavelength dependence of photoluminescence flickering in degraded MAPbI3 perovskite and its connection to lead iodide formation

Metal halide perovskite semiconductors often exhibit photoluminescence blinking and flickering when luminescence of individual small nano- or even microcrystals is monitored. The nature of these fluctuations is not well understood but must be related to the presence of metastable non-radiative recombination channels and efficient charge migration in these materials. Here we report on the excitation wavelength dependence of photoluminescence flickering effect in degraded methylammonium lead iodide (MAPbI3) thin films. While the luminescence intensity is temporary stable when excited in the blue region with wavelength shorter than 530 nm, excitation with red light (wavelength longer than 530 nm) results in luminescence flickering. It is hypothesised that the wavelength dependence reflects the excitation energy dependence of the photochemical mechanism that switches non-radiative recombination channels on and off. The effect can also be related to hindered charge carrier diffusion due to their localization in the interfacial layer between MAPbI3 and PbI2 which is formed in the course of degradation.

Tailoring spontaneous infrared emission of HgTe quantum dots with laser-printed plasmonic arrays

Chemically synthesized near-infrared to mid-infrared (IR) colloidal quantum dots (QDs) offer a promising platform for the realization of devices including emitters, detectors, security, and sensor systems. However, at longer wavelengths, the quantum yield of such QDs decreases as the radiative emission rate drops following Fermi’s golden rule, while non-radiative recombination channels compete with light emission. Control over the radiative and non-radiative channels of the IR-emitting QDs is crucially important to improve the performance of IR-range devices. Here, we demonstrate strong enhancement of the spontaneous emission rate of near- to mid-IR HgTe QDs coupled to periodically arranged plasmonic nanoantennas, in the form of nanobumps, produced on the surface of glass-supported Au films via ablation-free direct femtosecond laser printing. The enhancement is achieved by simultaneous radiative coupling of the emission that spectrally matches the first-order lattice resonance of the arrays, as well as more efficient photoluminescence excitation provided by coupling of the pump radiation to the local surface plasmon resonances of the isolated nanoantennas. Moreover, coupling of the HgTe QDs to the lattice plasmons reduces the influence of non-radiative decay losses mediated by the formation of polarons formed between QD surface-trapped carriers and the IR absorption bands of dodecanethiol used as a ligand on the QDs, allowing us to improve the shape of the emission spectrum through a reduction in the spectral dip related to this ligand coupling. Considering the ease of the chemical synthesis and processing of the HgTe QDs combined with the scalability of the direct laser fabrication of nanoantennas with tailored plasmonic responses, our results provide an important step towards the design of IR-range devices for various applications.

High‐Efficiency Formamidinium Lead Bromide Perovskite Nanocrystal‐Based Light‐Emitting Diodes Fabricated via a Surface Defect Self‐Passivation Strategy

AbstractFormamidinium lead bromide (FAPbBr3) nanocrystals (NCs) demonstrate great potential in light‐emitting diode (LED) applications due to their pure green emission and excellent stability. However, the abundant defects at the surface of the NCs act as charge trapping centers and significantly increase the trap‐assisted nonradiative recombination channels, hampering the performance improvement of LEDs based on FAPbBr3 NCs. Herein, a facile self‐passivation strategy of the surface defects is developed by introducing excess formamidinium bromide (FABr) during the colloidal synthesis of NCs, leading to much improved photoluminescence quantum yield (PLQY) of the obtained NCs. In addition, enhanced charge transport property is measured in the assembled films owing to the simultaneously declined insulating ligands at the surface of NCs. The molar ratio of FABr and PbBr2 is rationally optimized during the synthesis of NCs and high‐efficient green‐emissive LEDs are fabricated with a champion current efficiency of 76.8 cd A−1, corresponding to an external quantum efficiency of 17.1%, which is among the best‐performing green LEDs based on perovskite NCs so far.

The role of surface depletion layer effects on the enhancement of the UV emission in ZnO induced by a nanostructured Al surface coating

The UV enhancement of Al-coated ZnO single crystals with a wide range of carrier densities is systematically studied using depth-resolved cathodoluminescence and photoluminescence as well as valence band X-ray photoemission spectroscopy (VB-XPS). An up to 17-fold enhanced PL UV emission for Al-coated ZnO with the highest carrier density was measured, which falls to a 12-fold increase for the lowest carrier density. Depth-resolved cathodoluminescence measurements confirm that the enhancement is strongest near the Al-ZnO interface consistent with an increased UV emission due to an exciton-localized surface plasmon coupling mechanism. Correlative cathodoluminescence, photoluminescence and VB-XPS studies reveal that a number of additional effects related to the presence of the Al surface coating also contribute to the UV enhancement factor. These include increased UV enhancement due to the formation of a surface depletion layer induced by the Al coating, which also passivates competitive non-radiative surface recombination channels found in uncoated ZnO. Significantly, it was established that the magnitude of the emission enhancement factor can be raised in a controlled way by reducing the thickness of the depletion layer by increasing the carrier density. The contribution of these effects collectively provides an explanation for the large span of enhancement factors reported in the literature.

Suppressed Auger scattering and tunable light emission of Landau-quantized massless Kane electrons

The Landau level laser has been proposed a long time ago as a unique source of monochromatic radiation, widely tunable in the THz and infrared spectral ranges using an externally applied magnetic field. In spite of decades of efforts, this appealing concept never resulted in the design of a reliable device. This is due to efficient Auger scattering of Landau-quantized electrons, which is an intrinsic non-radiative recombination channel that eventually gains over cyclotron emission in all materials studied so far: in conventional semiconductors with parabolic bands, but also in graphene with massless electrons. The Auger processes are favored in these systems by Landau levels (or their subsets) equally spaced in energy. Here we show that this scheme does not apply to massless Kane electrons in gapless HgCdTe alloy, in which undesirable Auger scattering is strongly suppressed and the sizeable cyclotron emission observed, for the first time in the case of massless particles. The gapless HgCdTe thus appears as a material of choice for future technology of Landau level lasers.

Post‐Treatment of CH3NH3PbI3/PbI2 Composite Films with Methylamine to Realize High‐Performance Photoconductor Devices

Organometallic halide perovskites have attracted great research interest as light-active materials for use in optoelectronics. Here, we report a high-performance photoconductor based on a methylammonium lead iodide (MAPbI3 ) film that was prepared from a methylamine-treated MAPbI3 /PbI2 perovskite film. An ultrahigh responsivity of 3.6 A W-1 and detectivity of 5.4×1012 Jones were obtained for the film under 0.5 mW cm-2 white-light illumination. In addition, under 420 nm light irradiation, the film exhibited its highest responsivity and detectivity of 30 A W-1 and 2.4×1014 Jones, respectively. The excellent photo-response performance results from the improved electronic quality and suppressed nonradiative recombination channels of the treated perovskite thin film.

Beating the exclusion rule against the coexistence of robust luminescence and ferromagnetism in chalcogenide monolayers

Monolayer chalcogenide semiconductors with both luminescent and ferromagnetic properties are dreamed for simultaneous polarization and detection of the valley degree of freedom in valleytronics. However, a conventional chalcogenide monolayer lacks these coexisting properties due to their mutually exclusive origins. Herein we demonstrate that robust ferromagnetism and photoluminescence (PL) could be achieved in a (Co, Cr)-incorporated single monolayer MoS2, where the ferromagnetic interaction is activated by Co ions, and the nonradiative recombination channels of excitons is cut off by Cr ions. This strategy brings a 90-fold enhancement of saturation magnetization and 35-fold enhancement of PL intensity than the pristine MoS2 monolayer. The main reasons for the coexisting ferromagnetism and PL are the electronic interactions between the impurity bands of atop Cr adatoms and substitutional Co atoms, as well as the increased content of neutral exciton. Our findings could extend the applications of two-dimensional chalcogenides into spintronics, valleytronic and photoelectric devices.

High relative thermal sensitivity of luminescent molecular thermometer based on dinuclear [Eu2(mba)4(μ-mba)2(H2O)2] complex: The role of inefficient intersystem crossing and LMCT

Abstract In this work, the photophysical properties of the [Eu2(mba)4(μ-mba)2(H2O)2] is evaluated as a function of temperature. The luminescence intensity ratio (LIR) between the ligand emission and the 5D0 → 7F2 transition band area is analyzed giving a maximum relative sensitivity of 4.55% K−1 at 313 K and a temperature uncertainly smaller than 0.015 K over the operational range (203–313 K) making this complex as a promising molecular temperature optical probe. These results may be explained by the small energy difference between the excited singlet (S1) and triplet (T1) states (∼1400 cm−1) leading to an inefficient intersystem crossing and consequently the fluorescence of the ligand competing with the 5D0 state emissions. The Mott-Seitz model involving three non-radiative recombination channels suggests that the europium(III) emission deactivation involves the energy back transfer to ligand states, the ligand-to-metal charge transfer state and the third harmonic of the O-H oscillator from a coordinated water molecule.

Optical properties of porous Sm[formula omitted]-doped yttrium orthophosphate nanoparticles tailored by co-precipitation route

Sm3+ doped yttrium orthophosphate nanoparticles with porous structure was synthesized via simple co-precipitation method. The structure, the morphology and the elemental composition of the tailored nanoparticles were investigated using transmission electron microscopy (TEM), powder X-ray diffraction (XRD) and energy dispersive X-ray analysis (EDAX). Under the influence of trisodium citrate, Sm3+ doped YPO4 phosphor with porous structure could be synthesized. Under our reaction conditions, Sm3+ doped YPO4 nanophosphor crystallize in the hexagonal structure, as confirmed by the powder XRD patterns. The formation of porous structure turned out to be an essential step for the peculiar luminescent properties. The porosity undoubtedly enlarges the surface of the semiconductor/air interface, resulting in suppression of the non-radiative recombination channels. The tailored porous yttrium orthophosphate nanoparticles significantly displayed potentiality in the field of optical displays and also demonstrated the applicability in the biological field.

Terahertz Spectroscopy of Two-Dimensional Semimetal in Three-Layer InAs/GaSb/InAs Quantum Well

The features of terahertz photoluminescence and magnetoabsorption in magnetic fields up to 16 T in threelayer InAs/GaSb/InAs quantum wells with a band structure corresponding to a “two-dimensional semimetal” are studied. The comparison of the positions and amplitudes of the photoluminescence lines with the theoretical calculations of oscillator strengths for interband and intraband transitions performed using the eight-band Kane Hamiltonian indicates the existence of a nonradiative recombination channel associated with the overlap of the conduction and valence bands. The energies of interband and intraband Landau level transitions observed in the magnetoabsorption spectra are in good agreement with theoretical calculations, which confirms the predicted band structure.