Energy Up-conversion Process Research Articles

Unveiling a Counterintuitive Intermode Interplay in a Prototype Plasmonic Nanosystem.

We demonstrate a counterintuitive intermode interplay in the plasmonic system of gold nanorods, i.e., energy transfer (EnT) from the lower-energy longitudinal (L) mode to the higher-energy transverse (T) mode. The opening of this EnT(L→T) channel is enabled by an energy upconversion process with the L mode, in which the solvent environment plays a critical role. Switching from a strong thermal-conductivity solvent (i.e., water) to a much weaker one (i.e., chloroform) brings on prolongation of plasmonic hot-electron lifetime and enhancement of phonon emission, thereby increasing the probability of L-mode energy upconversion assisted by self-absorption of phonon emission. The pertinent justification and further manipulation of EnT(L→T) are provided by control experiments mainly from ultrafast spectroscopy. Besides, a subtle intermode dynamic screening effect in this unary plasmonic system is also addressed. This work refreshes our knowledge about the elusive intermode interplay in plasmonic systems and offers implementable strategies to harness hot electrons toward plasmon-mediated applications.

Net energy up-conversion processes in CdSe/CdS (core/shell) quantum dots: A possible pathway towards optical cooling

An investigation of the possibility of optical refrigeration (OR) on zinc-blende cadmium selenide/cadmium sulfide (CdSe/CdS) core/shell structure quantum dots (QDs) has been carried out. Quality samples were synthesized in our laboratory, and significant energy up-conversion photoluminescence (UCPL) was observed in these samples, showing the potential of generating net cooling effects. To better understand and predict the UCPL characteristics of the QDs, a semiempirical model has been developed, showing good agreement with our experimental results. The model takes into account the corresponding quantum yield and cooling efficiency, predicting the possibility of realizing optical refrigeration on a CdSe QD system.

Harvesting Sub-Bandgap IR Photons by Photothermionic Hot Electron Transfer in a Plasmonic p–n Junction

Plasmonic semiconductors are an emerging class of low-cost plasmonic materials, and the presence of a bandgap and band-bending in these materials offer new opportunities to overcome some of the limitations of plasmonic metals. Here, we demonstrate that in a plasmonic p-n heterojunction (Cu2-xSe-CdSe) the near-IR excitation (1.1 eV) of the hole plasmon in the p-Cu2-xSe phase results in rapid hot electron transfer to n-CdSe, with an energy 2.2 eV above the Fermi level. This hot electron generation and energy upconversion process can be well-described by a photothermionic mechanism, where the presence of a bandgap in p-Cu2-xSe facilitates the generation of energetic photothermal electrons. The lifetime of the transferred electrons in Cu2-xSe-CdSe can reach ∼130 ps, which is nearly 100× longer than that of its metal-semiconductor counterpart. This result demonstrates a novel approach for harvesting the sub-bandgap near IR photons using plasmonic p-n junctions and the potential advantages of plasmonic semiconductors for hot carrier-based devices.

Influence of ZnO nanoparticles on the performance of LED based on oligomer thin films

In this work, a set of thin films of ZnO nanoparticles doped with a conjugated oligomer {9,9,9′,9′,9″,9″-hexakis(hexyl)-2,7′;2′,7″-trifluorene} has been prepared. The optical and electrical measurements on pure oligomer, ZnO nanoparticles, and blended thin films have been investigated. The absorption spectra of the pure oligomer in thin films showed a single band at 350 nm. When the thickness was increased, there was no new band detected at the end of the spectrum. This is a strong indication that the oligomer cannot be present in the dimeric state. On the other hand, the photoluminescence (PL) spectra of the oligomer showed two distinguished peaks at 400 and 420 nm due to the monomer and excimer states, respectively. Furthermore, the results showed that the PL spectrum of the oligomer was affected by the thickness. For the blended thin films with different ratios of ZnO nanoparticles, the PL spectra showed an enhancement in intensity by increasing the concentration of ZnO nanoparticles. For the electrical properties, the addition of ZnO nanoparticles to oligomer had increased the current and luminescence. The luminescences of the pure and blend film at 10 V had reached 1000 and 4200 cd/m2, respectively; and the turn-on voltage was reduced from 10.5 to 5.8 V. The light emission in the device is due to the Auger-assisted energy up-conversion process. The lifetime of the blend’s diodes measured under standard atmospheric conditions was greatly improved.

All‐Solution‐Processed Pure Formamidinium‐Based Perovskite Light‐Emitting Diodes

All-solution-processed pure formamidinium-based perovskite light-emitting diodes (PeLEDs) with record performance are successfully realized. It is found that the FAPbBr3 device is hole dominant. To achieve charge carrier balance, on the anode side, PEDOT:PSS 8000 is employed as the hole injection layer, replacing PEDOT:PSS 4083 to suppress the hole current. On the cathode side, the solution-processed ZnO nanoparticle (NP) is used as the electron injection layer in regular PeLEDs to improve the electron current. With the smallest ZnO NPs (2.9 nm) as electron injection layer (EIL), the solution-processed PeLED exhibits a highest forward viewing power efficiency of 22.3 lm W-1 , a peak current efficiency of 21.3 cd A-1 , and an external quantum efficiency of 4.66%. The maximum brightness reaches a record 1.09 × 105 cd m-2 . A record lifetime T50 of 436 s is achieved at the initial brightness of 10 000 cd m-2 . Not only do PEDOT:PSS 8000 HIL and ZnO NPs EIL modulate the injected charge carriers to reach charge balance, but also they prevent the exciton quenching at the interface between the charge injection layer and the light emission layer. The subbandgap turn-on voltage is attributed to Auger-assisted energy up-conversion process.

Enhancing the electroluminescence of OLEDs by using ZnO nanoparticle electron transport layers that exhibit the Auger electron effect

ABSTRACTWe fabricated organic light-emitting diodes (OLEDs) with electron injection layers (EILs) and hole blocking layers (HBLs) made from different sized zinc oxide (ZnO) nanoparticles (NPs) made from indium tin oxide (ITO)/poly(3,4-ethylene dioxythiophene):poly (styrenesulfonate) (PEDOT:PSS)/poly(N′,N′-bis(4-butylphenyl)-N′,N′-bis(phenyl)-benzidine) (poly-TPD)/poly(9-vinylcarbazole) (PVK):tris(2-phenylquinoline)iridium(III) (Ir(2-phq)3)/ZnO NPs/Al. We made OLEDs with ZnO NPs with sizes of 5, 10, and 15 nm, and investigated the relationship between the size of the ZnO NPs and the luminance of the OLEDs. The luminance onset voltage is defined as the voltage required to generate a luminance of 1 cd/m2. The OLEDs with ZnO NPs with sizes of 5, 10, and 15 nm had luminance onset voltages of 3.1, 4.1, and 5.3 V, respectively, and luminances of the OLEDs with the ZnO NP sizes of 5, 10, and 15 nm were, respectively, 455.1, 194.4, and 34.8 cd/m2 at 8 V. The OLED with 5 nm ZnO NPs had the lowest luminance onset voltage and highest luminance. This indicates that the Auger electron effect which induces an energy up-conversion process occurred in the OLED with 5 nm ZnO NPs.

Spectral evidence for multi-pathway contribution to the upconversion pathway in NaYF4:Yb3+,Er3+ phosphors.

Although upconversion phosphors have been widely used in nanomedicine, laser engineering, bioimaging, and solar cell technology, the upconversion luminescence mechanism of the phosphors has been fiercely debated. A comprehensive understanding of upconversion photophysics has been significantly impeded because the number of photons incorporated in the process in different competitive pathways could not be resolved. Few convincing results to estimate the contribution of each of the two-, three-, and four-photon channels of near-infrared (NIR) energy have been reported in yielding upconverted visible luminescence. In this study, we present the energy upconversion process occurring in NaYF4:Yb3+,Er3+ phosphors as a function of excitation frequency and power density. We investigated the upconversion mechanism of lanthanide phosphors by comparing UV/VIS one-photon excitation spectra and NIR multi-photon spectra. A detailed analysis of minor transitions in one-photon spectra and luminescence decay enables us to assign electronic origins of individual bands in multi-photon upconversion luminescence and provides characteristic transitions representing the corresponding upconversion channel. Furthermore, we estimated the quantitative contribution of multiple channels with respect to irradiation power and excitation energy.

Numerical and experimental investigation of NIR-to-visible energy up-conversion in Er3+-doped sol–gel SiO2 powders

Er-doped SiO2 powders were prepared by a simple and straightforward sol–gel method using tetraethoxysilane and erbium triflate as sources of SiO2 and Er2O3. Excitation with near-infrared 800 and 980 nm light initiated an energy up-conversion process in these materials, which resulted in the emission in the predominantly red and green parts of the spectrum. Laser power dependence of the intensity of the up-converted emission was studied both experimentally and by using a numerical model to understand the up-conversion mechanism. Excited state absorption (ESA) and five unique ion–ion energy transfer up-conversion (ETU) processes between the relevant levels are identified as possible mechanisms for the observed luminescence. The efficient NIR-visible upconversion was observed even at comparatively low powers of 200 mW cm−2.

Multi-stacked organic light-emitting diodes using zinc oxide nanoparticle interfacial layers

Stacked organic light-emitting diodes (SOLEDs) with 30-nm nanoparticle (NP) interfacial layers were investigated. Zinc oxide (ZnO) was used as an interfacial layer between two green polymer (GP) layers. SOLEDs with NP interfacial layers had higher device efficiency than did a single-unit device due to the high probability of exciton recombination that originated from the Auger electron-assisted energy up-conversion process. Although the current density and luminance of SOLEDs with ZnO NP interfacial layers were smaller than those of the reference device, the efficiency was doubled because of the big band alignment difference and the large band gap between GP and ZnO NP interfacial layers, which induced more radiative-exciton recombination.

Phonon-assisted energy up-conversion process of [formula omitted] ions doped in [formula omitted] crystal

Er 3+ ions doped in KY F 4 crystal emit blue photoluminescence around 406 nm by energy up-conversion process when excited by 532 or 488 nm laser. By measuring the temperature dependence of up-converted luminescence, we found that the up-conversion process is enhanced with increasing temperature, which is an indication that phonons are assisting the process. This observation is supported by the fact that the energy transfer responsible for the up-conversion involves two transitions that have an energy mismatch.

Electroluminescence from light-emitting polymer/ZnO nanoparticle heterojunctions at sub-bandgap voltages

Summary The turn-on voltage of a light-emitting diode is generally equal to or greater than its bandgap voltage (or the bandgap energy divided by the electron charge). In contrast, we have found that electroluminescence at sub-bandgap voltages can be observed in polymer light-emitting devices with a heterojunction based on ZnO nanoparticles (NPs). This sub-bandgap electroluminescence is attributed to an Auger-assisted energy up-conversion process at the polymer/ZnO NPs interface, the observation of which depends strongly on the size of the nanoparticles.

Low-power green-to-blue and blue-to-UV upconversion in rigid polymer films

Green light excitation of platinum octaethylporphyrin (PtOEP) in poly(methyl methacrylate) (PMMA) films containing 9,10-diphenylanthracene (DPA) yielded emission of blue light under ambient conditions. This energy upconversion process was easily observed using low-power monochromatic excitation. The blue light emission arose from fluorescence of the lowest excited singlet state of DPA formed by spin-allowed annihilation of two DPA triplets excited by the transfer of the triplet energy from PtOEP to DPA. The upconversion emission intensity was proportional to the square of the excitation intensity, and an upconversion quantum efficiency of ∼0.0002 was measured at an excitation intensity of only 0.9 mW cm −2. High concentrations of molecularly doped emitter facilitated triplet energy migration in rigid PMMA. Upconverted fluorescence was also observed from other emitters and with other sensitizers, including blue-to-UV upconversion in a PMMA film containing the non-heavy metal sensitizer 2-methoxythioxanthone and the emitter 2,5-diphenyloxazole.

Photo-ESR and optical studies of Cr photoionization transition in ZnSSe:Cr crystals

Energy up-conversion in chromium doped ZnSe was related to two complementary 2+ → 1+ → 2+ ionization transitions of the chromium ion. Blue emission is observed with relatively high external quantum efficiency of energy up-conversion process (2×10−3 upon the green color optical pumping and at low temperatures). For Cr doped solid alloys of ZnSSe the energy of the pumping light can be tuned and the up-conversion process is still active, as we conclude from photoluminescence (PL) and photo-electron spin resonance (ESR) study.

Raman Microprobe Study of the Visible and Near-Infrared Excited Fluorescence Spectra of Glasses Examined as Potential Raman Intensity Calibration Standards

Abstract In previous work with the Raman microprobe, we have examined the fluorescence emissions of rareearth (RE) bearing glasses to study the phenomena of structural resonances in the spectra of glass microspheres and the energy upconversion processes for RE ions in such glasses that give rise to anti- Stokes fluorescence emission. Presently, we are revisiting these same fluorescent glasses to study their laser excited emissions as potential radiometric sources for the calibration of Raman spectral intensities. Many of the rare earth ions produce interesting fluorescence spectra on laser excitation depending on the type of ordered or disordered (solid) host matrix. These fluorescence spectra, depending on RE concentration and quenching effects, can be intense and may consist of either narrow-band (or line) emissions or broad-band spectra covering a wide spectral range. These spectra, acquired at laser wavelengths from the ultraviolet (UV) to the near-infrared (NIR), provide a practical means for the calibration of Raman instrumentation for both Raman frequency and intensity.

Orthogonal interaction of bernstein mode with ion-acoustic wave through plasma maser effect

Interaction of Bernstein mode wave with ion-acoustic turbulence is treated as the plasma maser effect. Ion-acoustic turbulence is considered as resonant mode, propagating orthogonal to the test Bernstein mode. The external magnetic field, in the direction of ion-acoustic turbulence, is playing key role in transferring energy in upconversion process from resonant low frequency wave to the high frequency wave through modulated electric field. It is shown that the direct coupling term and the associated reverse absorbtion effect do not contribute individually to the growth or damping to the Bernstein mode. Only the polarization coupling term is found to be effective in the energy upconversion process as the external magnetic field is providing momentum in the direction of propagation of Bernstein mode. The polarization coupling term is identified to be the dominant upconversion factor in the plasma maser effect.

Energy upconversion processes in Er 3+ and Nd 3+ doped RETiNbO 6 crystals

Single crystals of stoichiometric lanthanide rare earth compounds, RETiNbO 6, doped with Nd 3+ and Er 3+ ions have been grown by the laser heated pedestal growth technique. Excited state absorption resulting in energy upconverted fluorescence are observed for temperatures up to 298 K. The upconversion process is efficient because of weak nonradiative decay rate whilst energy transfer and concentration quenching processes are important in these materials.

Application of energy upconversion spectroscopy to novel laser and phosphor design

Energy-transfer and sequential-absorption upconversion processes, normally considered detrimental to conventional laser operation, are rapidly emerging as the key to an exciting new class of optical device. Energy-upconversion processes may be designed to take advantage of strong rare earth ion emission transitions in the visible region and of valuable co-incidences with diode-laser outputs in the near-infrared to produce compact visible lasers. This paper reviews the use of sequential-absorption and energy-transfer spectroscopy to derive information that is essential to the optimization of novel device designs as well as ‘conventional’ laser or phosphor designs. Transfer-rate extraction using upconverted temporal transients, determination of high-lying energy levels using sequential-absorption laser spectroscopy, ion dimer identification by line-narrowing experiments and direct identification of energy storage states using dual-laser techniques are presented. The use of dimer-dominated host crystals, such as CsCdBr 3, that can be used to isolate individual energy-transfer rates relevant to the optimization of many different laser types, is emphasized.

Electromagnetic emission in a magnetized plasma

We present a theory of emission of electromagnetic waves, in the ordinary and extraordinary modes, in the presence of Langmuir turbulence in a magnetized plasma due to a high frequency nonlinear force. The mechanism of emission considered is the plasma-maser interaction which is essentially an energy up-conversion process. The growth rates of the ordinary and extra-ordinary mode emissions are calculated and the results are compared with those obtained from the direct formulation. The scope of application of the results to radio spectra from solar flares is then stressed.

Plasma maser theory of the extraordinary mode in the presence of Langmuir turbulence

The emission of extraordinary mode radiation in a plasma with Langmuir turbulence driven by an electron beam is considered. The process of emission considered in this paper is the plasma maser effect, which is essentially an energy up-conversion process. The energy necessary for the growth of the extraordinary mode is derived from the Langmuir turbulence. The nonlinear dispersion relation of the extraordinary mode in the presence of Langmuir turbulence is obtained and its growth rate calculated. The scope of application of the results to space-plasma observation is then stressed.

The high-frequency nonlinear plasma instability

A simple derivation of the Langmuir wave growth rate in the presence of a coherent ion sound wave is shown, based on high-frequency nonlinear force. The new energy up-conversion process (plasma–maser) may play an important role in space and laboratory plasmas with enhanced low-frequency fluctuations.