Upconversion Enhancement Research Articles

Enhancing Multiphoton Upconversion from NaYF4:Yb/Tm@NaYF4 Core-Shell Nanoparticles via the Use of Laser Cavity.

We discover that emission efficiency of Tm3+-doped upconversion nanoparticles can be enhanced through the use of a laser cavity. With suitable control of the lasing conditions, the population of the intermediate excited states of the Tm3+ can be clamped at a required value above the excitation threshold. As a result, upconversion efficiency for the 300-620 nm emission band of the Tm3+-doped nanoparticles under 976 nm excitation can be enhanced by an order of magnitude over the case without a laser cavity. This is because the intrinsic recombination process of the intermediate excited states is suppressed and the surplus of excitation power directly contributes to the enhancement of multiphoton upconversion. Furthermore, our theoretical investigation has shown that the improvement of upconversion emission efficiency is mainly dependent on the cavity loss, so that this strategy can also be extended to other lanthanide-doped systems.

Plasmonically enhanced upconversion of 1500 nm light via trivalent Er in a TiO2 matrix

In this letter, we present a comparative experimental–simulation study of Au-nanodisc-enhanced upconversion of 1500 nm light in an Er3+ doped TiO2 thin film. The geometry of the Au nanodiscs was guided by finite-element simulations based on a single nanodisc in a finite computational domain and controlled experimentally using electron-beam lithography. The surface-plasmon resonances (SPRs) exhibited a well-known spectral red shift with increasing diameter, well explained by the model. However, an experimentally observed double-peak SPR, which resulted from inter-particle interactions, was expectedly not captured by the single-particle model. At resonance, the model predicted a local-field enhancement of the upconversion yield, and experimentally, the luminescence measurements showed such enhancement up to nearly 7 fold from a nanodisc with 315 nm diameter and 50 nm height. The upconversion enhancement agreed qualitatively with the theoretical predictions, however with 3–5 times higher enhancement, which was attributed to scattered light from neighboring particles.

Highly effective upconversion broad-band luminescence and enhancement in Dy2O3/Au and Sm2O3/Au composites

Upconversion (UC) nanomaterials with the ability to convert incident infrared photons to visible or ultraviolet emissions have attracted remarkable interest and hold great promising applications ranging from bioimaging to display devices. However, their low UCL efficiency does not satisfy the requirement of various practical applications. In this work, we successfully prepared Dy2O3/Au and Sm2O3/Au nanocomposites by a simple co-precipitation method. X-Ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and photoluminescence (PL) spectroscopy were used to characterize the samples. The UC broadband enhancements illuminating with various power density of 980nm are systemically investigated. The maximum UCL enhancement is 35-fold and 48-fold for Dy2O3/Au and Sm2O3/Au nanocomposites under 980nm excitation. The UC population processes and enhancement mechanism are carefully studied. The luminescence enhancement in the composites is mainly attributed to the enhanced thermal effect.

Unusually enhancing high-order photon avalanche upconversion of layered BiOCl:Er3 + semiconductor poly-crystals via Li+ ion intercalation doping

Efficient upconversion (UC) enhancement of lanthanide doped phosphors is highly desirable due to great application, but it might face severe challenges, especially for poly-crystal materials due to having relatively fewer defects and with less flexibility in material design. Here we present BiOCl semiconductor poly-crystals with two-dimensional structure, in which approximate 100-fold UC luminescence enhancement of Er3+ ion dopants can be readily achieved via intercalation doping of Li+ ions. As Li+ ions were incorporated into the interlayer of BiOCl crystals, unique internal electric field (IEF) in BiOCl that trigger unusual photon avalanche (PA) UC phenomena accompanying with excitation field enhancement of Er3+ ions can be improved significantly, leading to the excellent UC enhancement. These results may pave a novel way for constructing greatly efficient UC poly-crystal materials as well as enable an improved understanding for material structure to design efficient photonic materials.

Ultraviolet upconversion enhancement in triply doped NaYF4:Tm3+, Yb3+ particles: The role of Nd3+ or Gd3+ Co-doping

Upconversion (UC) particles are currently under intensive investigation, normally for their visible instead of ultraviolet (UV) light luminescence under near-infrared (NIR) irradiation. As a commonly studied host, NaYF4 in particular is known to have low phonon energy and high UC efficiency. Here, we present our work on enhancing UC luminescence in the UV region by adding a third dopant into a binary-doped NaYF4:Yb3+,Tm3+ host. More specifically, neodymium (Nd3+) or gadolinium (Gd3+) ions was co-doped into parent NaYF4:20mol%Yb3+,0.5mol%Tm3+ UC particles to enhance their UV UC luminescence. Experimental results demonstrated that these particles exhibited the highest degree of UV UC enhancements when co-doped with 0.05mol% Nd3+ or 2.0mol% Gd3+, expanding the potential of this type of materials into many possible applications by directly converting NIR irradiation into UV light. Fundamentally, the UV UC emission dependence of these triply doped NaYF4:Yb3+,Tm3+ particles with different Nd3+ and Gd3+ doping concentrations was investigated in terms of ground state absorption, excited state absorption and energy transfer UC mechanisms.

Observation of Considerable Upconversion Enhancement Induced by Cu2-xS Plasmon Nanoparticles.

Localized surface plasmon resonances (LSPRs) are achieved in heavily doped semiconductor nanoparticles (NPs) with appreciable free carrier concentrations. In this paper, we present the photonic, electric, and photoelectric properties of plasmonic Cu2-xS NPs/films and the utilization of LSPRs generated from semiconductor NPs as near-infrared antennas to enhance the upconversion luminescence (UCL) of NaYF4:Yb(3+),Er(3+) NPs. Our results suggest that the LSPRs in Cu2-xS NPs originate from ligand-confined carriers and that a heat treatment resulted in the decomposition of ligands and oxidation of Cu2-xS NPs; these effects led to a decrease of the Cu(2+)/Cu(+) ratio, which in turn resulted in the broadening, decrease in intensity, and red-shift of the LSPRs. In the presence of a MoO3 spacer, the UCL intensity of NaYF4:Yb(3+),Er(3+) NPs was substantially improved and exhibited extraordinary power-dependent behavior because of the energy band structure of the Cu2-xS semiconductor. These findings provide insights into the nature of LSPR in semiconductors and their interaction with nearby emitters and highlight the possible application of LSPR in photonic and photoelectric devices.

Local Field Modulation Induced Three-Order Upconversion Enhancement: Combining Surface Plasmon Effect and Photonic Crystal Effect.

A 2D surface plasmon photonic crystal (SPPC) is achieved by implanting gold nanorods onto the periodic surface apertures of the poly(methyl methacrylate) (PMMA) opal photonic crystals. On the surface of the SPPC, the overall upconversion luminescence intensity of NaYF4 :Yb(3+) , Er(3+) under 980 nm excitation is improved more than 10(3) fold. The device is easily shifted to a transparent flexible substrate, applied to flexible displays.

Fabrication of core@spacer@shell Aunanorod@mSiO2@Y2O3:Er nanocomposites with enhanced upconversion fluorescence

We fabricated the well-defined Aunanorod@mSiO2@Y2O3:Er nanocomposites with about 10- and 8-fold strongest upconversion enhancement for green and red emissions, respectively.

Tens of thousands‐fold upconversion luminescence enhancement induced by a single gold nanorod

AbstractUpconversion nanoparticles (UCNPs) have gained increasing attention for their wide applications in bioimaging, displays and photovoltaics. However, low efficiency has been an ongoing challenge for further developments. In this work, it is proposed that the ultrasmall size of UCNPs is essential for achieving large enhancement factors and experimentally demonstrated with 4‐nm UCNPs. A strategy of plasmonic dual resonance is proposed in which two distinct localized surface plasmon resonance (LSPR) peaks of gold nanorods (GNRs) were designed to perfectly match both the excitation and emission light wavelength of UCNPs. Combining the excitation enhancement and Purcell effect, a huge enhancement factor of tens of thousands‐fold is stochastically demonstrated for single UCNPs in solution. The largest overall enhancement region is close to the end of a GNR but not in its central part. The excitation enhancement (up to three orders of magnitude) and the emission enhancement (larger than one order of magnitude) induced by the Purcell effect are experimentally demonstrated separately. This study provides insight into how to achieve a very large upconversion enhancement factor with surface plasmons and will catalyze development of UCNPs’ extensive applications.image

Tailoring Plasmonic Enhanced Upconversion in Single NaYF4:Yb(3+)/Er(3+) Nanocrystals.

By using silver nanoplatelets with a widely tunable localized surface plasmon resonance (LSPR), and their corresponding local field enhancement, here we show large manipulation of plasmonic enhanced upconversion in NaYF4:Yb3+/Er3+ nanocrystals at the single particle level. In particular, we show that when the plasmonic resonance of silver nanolplatelets is tuned to 656 nm, matching the emission wavelength, an upconversion enhancement factor ~5 is obtained. However, when the plasmonic resonance is tuned to 980 nm, matching the nanocrystal absorption wavelength, we achieve an enhancement factor of ~22 folds. The precise geometric arrangement between fluorescent nanoparticles and silver nanoplatelets allows us to make, for the first time, a comparative analysis between experimental results and numerical simulations, yielding a quantitative agreement at the single particle level. Such a comparison lays the foundations for a rational design of hybrid metal-fluorescent nanocrystals to harness the upconversion enhancement for biosensing and light harvesting applications.

Importance of suppression of Yb(3+) de-excitation to upconversion enhancement in β-NaYF4: Yb(3+)/Er(3+)@β-NaYF4 sandwiched structure nanocrystals.

Nanosized Yb(3+) and Er(3+) co-doped β-NaYF4 cores coated with multiple β-NaYF4 shell layers were synthesized by a solvothermal process. X-ray diffraction and scanning electron microscopy were used to characterize the crystal structure and morphology of the materials. The visible and near-infrared spectra as well as the decay curves were also measured. A 40-fold intensity increase for the green upconversion and a 34-fold intensity increase for the red upconversion were observed as the cores are coated with three shell layers. The origin of the upconversion enhancement was studied on the basis of photoluminescence spectra and decay times. Our results indicate that the upconversion enhancement in the sandwiched structure mainly originates from the suppression of de-excitation of Yb(3+) ions. We also explored the population of the Er(3+4)F9/2 level. The results reveal that energy transfer from the lower intermediate Er(3+4)I13/2 level is dominant for populating the Er(3+4)F9/2 level when the nanocrystal size is relatively small; however, with increasing nanocrystal size, the contribution of the green emitting Er(3+4)S3/2 level for populating the Er(3+4)F9/2 level, which mainly comes from the cross relaxation energy transfer from Er(3+) ions to Yb(3+) ions followed by energy back transfer within the same Er(3+)-Yb(3+) pair, becomes more and more important. Moreover, this mechanism takes place only in the nearest Er(3+)-Yb(3+) pairs. This population route is in good agreement with that in nanomaterials and bulk materials.

Simultaneous size and luminescence control of NaYF4:Yb3+/RE3+ (RE = Tm, Ho) microcrystals via Li+ doping

Enhancement of upconversion (UC) luminescence is imperative for the applications of UC microcrystals (MCs). In this work, NaYF4:Yb3+/RE3+ (RE=Tm, Ho) MCs via Li+ doping were successfully prepared by a simple hydrothermal process with the assistance of citric acid. The UC luminescence intensities of NaYF4:Yb3+/RE3+ (RE=Tm, Ho) are significantly enhanced via Li+ doping at different concentrations. Compared to Li+-absent sample, UC luminescence intensities of blue emission (477nm) and red emission (649nm) in NaYF4:Yb3+/Tm3+ MCs via 15mol% Li+ doping are improved by 10 and 9 times, respectively; UC luminescence intensities of green emission (538nm) and red emission (644nm) in NaYF4:Yb3+/Ho3+ MCs via 15mol% Li+ doping are improved by 12 and 3 times, respectively. The mechanism of the enhancement via Li+ doping is discussed in details, which may be attributed to the fact that Li+ doping can cause the distortion of the local symmetry around RE ions. Our results indicate that the enhanced UC luminescence of NaYF4:Yb3+/RE3+ (RE=Tm, Ho) MCs via Li+ doping may have potential applications in optoelectronic devices such as solar cells and plasma display panel.

Up-conversion enhancement in Er3+ doped TiO2 through plasmonic coupling: Experiments and finite-element modeling

Silver nanoparticles, placed on top of an Er3+ doped TiO2 thin film, were investigated as a means of achieving enhanced up-conversion. Finite-element modeling was used to determine the nanoparticle dimensions (height and diameter) yielding the largest plasmonic enhancement for an incident light wavelength of 808 nm. In order to mimic the experimentally observed up-conversion enhancement, the electric-field enhancement from the Ag nanoparticles was integrated over the entire thickness of the thin film. Based on these calculations, four samples were prepared and tested. The trends predicted by the models were found to correlate well with the trends of the experimentally obtained plasmonic enhanced up-conversion yields. The largest plasmonic enhancement for 808 nm excitation was observed for Ag nanoparticles of diameter 91 ± 5 nm and height 14 ± 1 nm, yielding 163- and 51-fold enhancements for the green (525 nm and 550 nm) and red (660 nm) emissions peaks, respectively.

Upconversion Luminescence of Rare-Earth-Doped Y2O3 Nanoparticle with Metal Nano-Cap

The upconversion property of an individual composite nanoparticle consisting of a metal (Ag) nanocap and a rare-earth doped upconversion nanoparticle (Er- and Yb-doped Y2O3 nanoparticle) was studied. The structural parameters of the composite nanoparticle were chosen so that the resonant wavelengths of the electric dipole and magnetic dipole surface plasmon modes of a nanocap coincide with the upconversion luminescence peaks of Er3+. Strong modification of the upconversion spectrum was observed by the formation of a Ag nanocap. Upon excitation at 980 nm, the green (∼550 nm) and red (∼670 nm) peaks were on average 23 and 48-fold, respectively, enhanced. The strong modification of the spectral shape, i.e., the intensity ratio of the green to red luminescence, suggests that the enhancement of radiative decay rates by the two surface plasmon modes is mainly responsible for the upconversion enhancement.

Decrease in particle size and enhancement of upconversion emission through Y3+ions doping in hexagonal NaLuF4:Yb3+/Er3+nanocrystals

The upconversion (UC) enhancement with size decrease has been realized through Y3+ ions doping in β-NaLuF4:Yb3+/Er3+ nanocrystals (NCs) by a solvothermal process. X-Ray diffraction (XRD), scanning electron microscopy (SEM) and UC luminescence spectra were used to characterize the resulting samples. With an increase of the Y3+ doping concentration, the NCs size is continuously decreased with no phase transition. Meanwhile, the UC intensity is firstly increased and then it decreases when the Y3+ ions concentration is over 30 mol%. The NCs (β-NaLu0.48Y0.3Yb0.2Er0.02F4) with the optimum Y3+ ions doping concentration (30 mol%) for UC emission have a diameter of about 80 nm. Compared with β-NaLu0.78Yb0.2Er0.02F4 and β-NaY0.78Yb0.2Er0.02F4 prepared under the same conditions, the green UC emission is enhanced by a factor of 1.8 and 16, respectively, for β-NaLu0.48Y0.3Yb0.2Er0.02F4. The variation of the UC intensity with the increasing Y3+ ions concentration is attributed to the changing of the symmetry of the local crystal field induced by Y3+ ions doping, which has been proved by the structural probe Eu3+ ions. For β-NaLu0.78Yb0.2Er0.02F4 and β-NaLu0.48Y0.3Yb0.2Er0.02F4, a three-photon green UC process and a three-photon red UC process occur simultaneously at high pump power density, which can be described as 4G11/2 (Er3+) + 4I15/2 (Er3+) → 2H11/2/4S3/2 (Er3+) + 4I13/2 (Er3+) and 4G11/2 (Er3+) + 2F7/2 (Yb3+) → 4F9/2 (Er3+) + 2F5/2 (Yb3+), respectively.

Upconversion enhancement of lanthanide-doped NaYF4 for quantum dot-sensitized solar cells

As an upconverter, lanthanide-doped NaYF4 (Ln-NaYF4) has been integrated into the photoanodes for quantum-dot sensitized solar cells. Compared with the solar cells with pure TiO2, the upconverter based solar cells exhibited a 17.6% increase in the photocurrent and a 20% increase in the power conversion efficiency. Furthermore, when the integrated Ln-NaYF4 is pretreated by annealing, the resulting photoanode demonstrates an obvious improvement in the photocurrent generation and the power conversion efficiency due to the annealing-induced enhancement of the upconversion emission as well as the elimination of the defects allowing for efficiently reducing charge recombination. The best photovoltaic performance with a short-circuit current of 14.36mA/cm2 and a power conversion efficiency of 4.37% has been obtained with the solar cells containing the annealed upconverters. Both the solar cells integrated with the Ln-NaYF4 pretreated by annealing or not demonstrate obvious photocurrent responses under the illumination of the NIR light. This finding demonstrates the feasibility of the combination of the up-converting materials with quantum dot-sensitized solar cells to extend the exploitation of the solar light to a wider range thus enabling the fabrication of highly efficient solar cells.

Effect of Copper nanoparticles on the enhancement of upconversion in the Tb3+/Yb3+ co-doped transparent glass–ceramics

The transparent glass–ceramics SiO2–AlF3–BaF2–TiO2–CaCO3 containing BaF2 nanocrystals were successfully prepared by conventional melting method. The effect of CuNPs on the enhancement upconversion emission intensity in the Tb3+/Yb3+ co-doped transparent glass–ceramics under 980nm laser diode excitation were investigated. The structural investigation carried out by X-ray diffraction (XRD) and transmission electron microscopy (TEM) evidenced the formation of cubic BaF2 nanocrystals. The distribution of Copper nanoparticles and the average diameter size which were confirmed by the results of XRD, TEM measurements. Compared with the glass–ceramics sample without Cu2O element, the intensity of upconversion emission was significantly increased. Particularly, the red UC emission intensity bands at 588 and 624nm strongly increased with the increase of Cu2O concentration and reached its maximum at 2.5mol.%, then decreases due to the self-quenching effect. The possible mechanism responsible for the enhancement of upconversion emission intensity in the Tb3+/Yb3+ co-doped due to effectiveness of surface plasmon resonance was discussed.

Effects of gold nanoparticles on the enhancement of upconversion and near-infrared emission in Er3+/Yb3+ co-doped transparent glass–ceramics containing BaF2 nanocrystals

The transparent glass–ceramics SiO2–AlF3–BaF2–TiO2–CaCO3 containing BaF2 nanocrystals were successfully prepared by the conventional melting method. The effect of the AuNPs on the enhancement upconversion and near-infrared emission in Er3+/Yb3+ co-doped transparent glass–ceramics under 980nm laser diode excitation were investigated. The transmission electron microscopy (TEM) micrograph showed the average diameter size of gold nanoparticles is estimated about 4–6nm. Compared with the glass–ceramics sample without HAuCl4, the upconversion and near-infrared emissions intensity of SAB glass–ceramics samples with HAuCl4 were significantly increased. Particularly, the red UC emission intensity band centered at 850 nm strongly increased about twenty-fold with the increase of HAuCl4 concentration and reaches its maximum at 1.5mol%, then decreases due to the self-quenching effect. The possible mechanism responsible for the enhancement of upconversion and near-infrared emission in Er3+/Yb3+ co-doped due to effectiveness of surface plasmon resonance were discussed.

Li+ ions doping core–shell nanostructures: An approach to significantly enhance upconversion luminescence of lanthanide-doped nanocrystals

Currently, enhancing luminescence efficiency of upconversion nanoparticles (UCNPs) has still been a challenging topic in despite of many efforts to improve UC luminescence. In this paper, a new strategy to enhance UC emission has been described based on coupling of Li+ ions doping with growth an inert shell. Significant enhancement of upconversion (UC) luminescence intensity of lanthanide-doped nanoparticles was observed under synergistic effect of internal adjustment and external approach. In addition, the mechanism for the luminescence enhancement of UCNPs has been discussed. Our results suggest that this UC enhancement strategy, proven here in NaGdF4 nanoparticles, could be extended to other lanthanide-doped nanocrystal systems for applications ranging from luminescent biological labels to volumetric three-dimensional displays.

Plasmon enhancement mechanism for the upconversion processes in NaYF4:Yb(3+),Er(3+) nanoparticles: Maxwell versus Förster.

Rare-earth activated upconversion materials are receiving renewed attention for their potential applications in bioimaging and solar energy conversion. To enhance the upconversion efficiency, surface plasmon has been employed but the reported enhancements vary widely and the exact enhancement mechanisms are not clearly understood. In this study, we synthesized upconversion nanoparticles (UCNPs) coated with amphiphilic polymer which makes UCNPs water soluble and negatively charged. We then designed and fabricated a silver nanograting on which three monolayers of UCNPs were deposited by polyelectrolyte-mediated layer-by-layer deposition technique. The final structures exhibited surface plasmon resonance at the absorption wavelength of UCNP. The green and red photoluminescence intensity of UCNPs on nanograting was up to 16 and 39 times higher than the reference sample deposited on flat silver film, respectively. A thorough analysis of rate equations showed that the enhancement was due entirely to absorption enhancement in the strong excitation regime, while the enhancement of both absorption and Förster energy transfer contribute in the weak excitation regime. The Purcell factor was found to be small and unimportant because the fast nonradiative decay dominates the relaxation process. From the experimentally observed enhancements, we concluded 3.1× and 1.7× enhancements for absorption and Förster energy transfer, respectively. This study clearly shows the plasmon enhancement mechanism and its excitation power dependence. It provides the basis for comparison of the enhancements of various plasmonic UCNP systems in the literature. It also lays the foundation for rational design of optical plasmonic structures for upconversion enhancement.