Special optical performance from single upconverting micro/nanoparticles

In 1966, Francois Auzel observed upconverted visible (vis) emission from rare-earth (RE) ions for the first time.¹ Since then, upconversion (UC), known as an anti-Stokes emission process, continues to intrigue researchers in various fields. Photon UC is a nonlinear optical phenomenon, in which RE ions sequentially absorb two or more low-energy photons and then produce high-energy photon emission, resulting in near-infrared (NIR)-excited vis, and even ultraviolet (UV) emission. In addition, due to its unique advantages, such as multiple-peak spectral patterns, long lifetime, exceptional photostability, absence of on-off blinking and photobleaching, deep excitation penetration, and no overlap with cellular autofluorescence,^2–5 UC from RE-based micro/nanoparticles presents potential applications in three-dimensional (3D) volumetric displays, fluorescence microscopy imaging, nanoscale sensors, security inks, nanolasers, solar cells, biomedical fields, etc.^6–14 

The past 20 years have witnessed a rapid development of UC micro/nanoparticles (UCM/NPs). From 2004 to 2012, material scientists made great advances in the fabrication techniques, UC mechanisms, and optimization strategies for brightening UC emission.¹⁵ Since 2010, precise controls over the morphology and colloidal monodispersity of UCM/NPs,^16–18 as well as their internal energy transfer mediated optical performance, have been realized,¹⁹ and considerable efforts have been devoted to developing UC nanomaterials for a wide range of emerging applications. However, most investigations about the UCM/NPs focused on the ensemble optical performance, while less effort has been devoted to the measurement of a single particle. In fact, single UCM/NPs obtained in the same synthesis process could be different in terms of size, shape, defects, surface groups, and charges. These are core issues in fundamental research related to materials science, crystal structure, and interfacial chemistry.²⁰ Meanwhile, they are important for reproducibility, functionality, and applications.²⁰ For example, the vis emission spectra of approximately 40 individual 8 nm UCNPs show different spectral features compared with the room-temperature spectra of larger UCNPs. Their high-resolution spectra differ from particle to particle, possibly resulting from heterogeneity of their surface characteristics or disorder in the distribution of RE ions within the nanoparticles.^21,22 Besides, based on the measurement of a single UC particle, it has been proved that the concentration quenching does not limit the sensitizer concentration. The surface quenching and large size are the key factors limiting the optimal sensitizer concentration.²³ Hence, precise knowledge about the optical performance of a single UCM/NP is extremely important for the optimization of UC RE-based materials and for fully exploiting their capability in interdisciplinary applications. In recent years, owing to the development of sophisticated optical microscopy technology, a growing number of studies focused on the characterization of single UCM/NPs, resulting in the observation of optical features that are unknown before. Herein, this Perspective presents recent advances in studying the properties of single UCM/NPs, including spectral features of a single UC particle and the relevant applications in different fields.

In the optical performance study of a single particle, morphology, size, monodispersity, and structure are equally important, which determine whether the as-prepared micro/nanoparticles are suitable for single-particle level characterization. Ex situ transmission and scanning electron microscopy (TEM and SEM) are common tools that are used to determine morphology, size, and monodispersity [Fig. 1(a)]. When combined with an energy-dispersive x-ray spectrometer, the composition and elemental distribution in a single particle can be obtained. Atomic-force microscopy (AFM) can provide morphology information of a single particle yet in an ambient atmosphere. More importantly, it can manipulate the two-dimensional (2D) orientation of a single particle through an AFM tip [Fig. 1(b)]. Due to the small-size UCM/NPs, a few tools are available for obtaining the structural information of a single particle. Raman scattering is strongly dependent on the sample polarizability and ion coordination polyhedra.^24–26 Thus, each structure has corresponding characteristic Raman peaks, and the structural information of the sample can be obtained based on the detected Raman spectra. With the assistance of the microscopy technique, it is possible to excite single particles and collect their Raman scattering. Accordingly, Raman spectra make the single-particle structural characterization possible [Fig. 1(c)]. In our previous study, the structural anisotropy of Yb³⁺/Pr³⁺ co-doped a single hexagonal (β) NaYF₄ microrod was determined by Raman spectra.²⁷ Moreover, high-resolution TEM lattice fringe and selected area electron diffraction also provide structural information of a single particle.

Single-particle optical characterization allows for the exploration of the crystalline structure-dependent optical differences and some unexpected unique optical phenomena at the sub-micron and even nano-scale. Up to now, there are two mainstream approaches used for single UCM/NPs' optical characterization: fiber-loaded measurement and confocal scanning microscopy.

Fiber-loaded measurement employs a hollow microstructured optical fiber to capture nanoparticles through capillary force for detection [Fig. 1(d)]. The NIR excitation laser is coupled to the loaded fiber with the help of a dichroic mirror and a microscope objective, propagating along the fiber to excite the trapped single UCM/NPs. The UC emissions from single UCM/NPs are collected by the same loaded fiber and propagate backward. After the light guided by the fiber propagates through the dichroic filter and broad bandpass filter, residual backscattered pump light is removed, and the UC emissions are detected by a spectrometer. In this measurement, the UCM/NPs need to be dispersed in a nonpolar solvent, such as toluene and cyclohexane, and the dispersion needs to be diluted to a specific concentration, ranging from tens of fM to tens of nM.^28,29 The optical performance characterization of single UCM/NPs using liquid-immersed exposed-core microstructured optical fibers has been reported by Monro et al.^30–32 

Figure 1(e) shows the schematic of confocal scanning microscopy. After passing a bandpass filter, the NIR excitation laser beam propagates through a dichroic mirror and is focused onto the sample by an objective. The UC emissions are collected through the same objective and then reflected by the dichroic mirror to remove the excitation light. Finally, after passing through a filter, the UC emissions are captured by a spectrometer. For this measurement, the UCM/NPS should have good monodispersity so that inter-particles are large enough for single-particle detection. Compared with other approaches, confocal scanning microscopy has greater flexibility. For instance, adjusting the piezo-actuated 3D nano-positioning stage could locate different single particles under identical test conditions, and it could be possible to excite different positions of a single particle and to excite a single particle from different angles. Meanwhile, equipped with different detectors, it could obtain more optical information from a single particle, such as fluorescence imaging and lifetime.³³ Furthermore, combining with external fields could enable the observation of single-particle optical performance in different environments.^34–37 Therefore, in recent studies, confocal scanning microscopy becomes increasingly popular in the community of UC materials.

Single-particle spectroscopy is a powerful technique to explore microstructure-dependent spectroscopic characters, which are always compromised in ensemble spectroscopy. Attributed to the rapid development of the single-particle spectroscopic technique, a growing number of optical features have been observed in single UCM/NPs, thereby greatly extending the application of UC materials in different fields. In the following, we would like to introduce important recent developments that are devoted to the optical behaviors of single UCM/NPs.

It is well known that UC is a typical nonlinear luminescence process [Fig. 2(a)]. This process is very different between ensemble spectroscopy and single-particle spectroscopy. It has been concluded from ensemble UC optical studies that optimal doping concentrations of Tm³⁺ and Er³⁺ should be less than 1 mol. % and 2 mol. % in the β-NaYF₄ host, respectively, because of concentration quenching.³⁸ However, in a single UC particle, the concentration quenching is highly dependent on the excitation power.^21,28 After increasing the excitation power density to 10⁴W⋅cm⁻² or higher, 8 mol. % Tm³⁺ and 20 mol. % Er³⁺ highly doped-single UCNPs are orders of magnitude brighter than conventional lightly doped UCNPs.^21,28 When inserting a passivation layer to construct a core–shell–shell design, the single NaYbF₄:8%Er³⁺ nanoparticle has been reported to show high brightness and emit about 200 photons per second under a low excitation power density of 8 W cm⁻².³⁹ These findings indicate that highly RE ion-doped UCNPs at sufficient irradiance excitation have large potential for single-particle tracking and remote sensing, especially as photostable, background-free and extremely bright labeling probes in bioimaging.

The “bright” and “dark” spots representing different UC efficiency in a single UCM/NP may coexist in an intensity mapping of UC emission [Fig. 2(b)]. This is a clear signal that some unknown defects may be induced during synthesis, thereby leading to inconsistent crystalline symmetries among UCM/NPs although they have similar morphology. This may also imply that different UCM/NPs lying with different lattice planes on the surface have a specific atomic density and orientations when excited with a polarized laser beam.²⁹ Using a confocal microscopy system, Zheng and co-workers investigated UC photoluminescence (PL) spectra and fluorescence photos of single LiYF₄ microcrystals doped with different RE ions and obtained important spectral features that are difficult to obtain via ensemble spectroscopy.^40–42 With the increase in crystal size, single doped LiYF₄ microcrystals with similar morphology present decreased UC emission intensity, indicating that organic ligands introduced by organic additives affect the emission property through changing the nonradiative relaxation rate.⁴⁰ When exciting different positions of a single doped LiYF₄ microcrystal, the different emission intensities are correlated with the difference in the local site symmetry of the crystal field.⁴¹ These conclusions from the precise single-particle spectroscopic study are of great importance to improve the UCM/NP’s quality, which however are ignored in ensemble measurements.

Low-dimensional micro/nanomaterials with highly anisotropic optoelectronic properties play key roles in a broad range of electronic and photonic applications.^43–45 A main manifestation of the anisotropic optoelectronic properties is remarkable optical polarization characteristics in terms of emission, absorption, and photoconductivity.⁴⁶ Recently, due to potential applications in chip-level polarization sensitive devices, microscopic multi-information transformation, phase transitions detection in nanomaterials, biomedical imaging devices as well as optical storage, encryption, sensing, and full-color display, optical polarization characteristics from low-dimensional micro/nanomaterials have attracted growing attention.^27,47–53 RE ions with multiple-peak emission patterns also exhibit remarkably UC light polarization characteristics in the anisotropic structures, which is manifested by the periodic change in UC emission intensity when rotating the polarization angle [Fig. 2(c)].

The UC light polarization characteristics from RE ions are related to the crystal orientation. For example, in a single β-NaYF₄ microrod, when excitation light propagates perpendicularly to the crystal c axis with a polarization perpendicular to the a axis, the UC luminescence presents obvious polarization characteristic. In comparison, when the excitation light propagates perpendicularly to the crystal a axis, the UC emission intensity remains almost unchanged along with the variation of excitation polarization, suggesting that there is no polarization dependence in the UC emission.²⁷ Also, in a single uniaxially anisotropic tetragonal-phase LiYF₄ microcrystal, Er³⁺ presents different UC emission polarization degrees when changing the crystal orientation by optical trapping.⁵⁴ Therefore, UC light polarization characteristics cannot be observed by ensemble measurements owing to the random orientation of particles.⁵⁵ In our previous work,²⁷ based on density functional theory calculations and Raman spectra measurement, it is found that the UC light polarization characteristics of RE ions originate from the anisotropic microstructure of single UCM/NPs and we established the relationship between anisotropic microstructure and optical properties through an individual particle. Zhou et al.⁵⁶ compared the UC emission polarization characteristics between Yb³⁺/Tm³⁺ co-doped single β-NaYF₄ nanorods and microdisks and found that there is nearly no difference, suggesting that the light polarization characteristics of RE ions are dominated by the selection rules for local site symmetries as well as by the crystal symmetry, and not by the shape and size of the particle. Exploiting polarization characteristics of the UC emission, potential applications in microscopic multi-information transportation are suggested for single UCM/NPs. Rodríguez-Sevilla et al.⁵⁰ used polarized spectroscopy from a single UC nanoparticle to determine its orientation dynamics when entering an optical trap and further successfully determine the intracellular dynamic viscosity.

It is very important to direct the propagation of light on the wavelength scale for applications, such as high-contrast displays, microlasers, and integrated photonics devices.^57–60 Compared with coupling light into waveguides, localized absorption of the incident light to produce directional optical emissions is a promising alternative.^61–64 There have been various luminescent waveguides based on UV radiations.^65–67 However, directional emissions under NIR radiation in a waveguide remain unexplored, although NIR radiations show long penetration depth and less destructive and are preferable for biology applications.^68,69 Chen et al.⁷⁰ found β-NaYF₄ with a refractive index of 1.43 is larger than that of air. This will lead to the total internal reflection of the upconverted light at the microcrystal–air interface, forming the waveguide effect [Fig. 2(d)]. As a proof-of-concept demonstration, they employed β-NaYF₄ microrods co-doped with Yb/Er (10/1 mol. %) to generate NIR-excited (976 nm) UC emission waveguide.⁷⁰ When the excitation laser is uniformly projected onto a single microrod, brighter UC emissions were detected at both ends of the microrod. When a convergent laser beam is focused on the central part of a single microrod, UC emissions were detected in both ends of the microrod. By shifting the focused laser beam to one end of the single microrod, bright emissions appeared at both ends. These observations indicate that a single doped β-NaYF₄ microrod absorbs NIR light, and the UC emissions propagate within the rod directionally. Accordingly, single UCM/NPs provide a possibility for NIR-excited waveguides. Gao et al.⁷¹ found an additional advantage of the waveguide effect from single UCM/NPs. When confining the luminescence propagation along the surface of the microcrystal, there is a super-bright local luminescence at the tail end, which is because this process is prone to interference and diffraction that redistributes the spatial intensity of light. Therefore, different luminescent patterns from a series of RE ion-doped single UCM/NPs were obtained, including a dual-color dumbbell, sunflower, rod-shaped candy, dichroic plate, and colored ring.^71,72 These results might be exploited for multicolor display. Except for the UC emissions, single UCM/NPs also can propagate NIR excitation light directionally. Using waveguiding-excitation, the overall integrated UC intensity of a single UCM/NP is 10-fold stronger compared with the case of the spot-excitation approach.⁷¹ 

The multiple 4f energy levels of the trivalent RE ions make it possible to establish population inversion and amplified stimulated emission at a relatively low pump power.⁷³ As mentioned in the Waveguide part, UC emission occurs total reflection at the microcrystal–air interface of a single UCM/NP. Therefore, the single UCM/NP can be used as a gain medium or as a laser cavity to achieve resonance⁷⁴ for the demonstration of the UC micro/nano-laser. Wang et al.⁷⁵ prepared Yb³⁺–Er³⁺–Tm³⁺ tri-doped single β-NaYF₄ microrods. The single β-NaYF₄ microrod supports the whispering-gallery-mode (WGM) resonance owing to the waveguide effect. With sensitization by Yb³⁺, red, green, and blue (RGB) UC emissions from Er³⁺ and Tm³⁺ contribute to the optical gain in different spectral regions. Finally, excited by a 980 ns-pulsed NIR laser, a single-mode UC white-light laser was obtained [Fig. 1(e)]. Notably, to realize the white-light laser, the three dopant concentrations and the microrod size are crucial. The former ensures RGB emissions simultaneously producing similar emission intensity, and the latter ensures the threshold of all the RGB WGMs in a close range of pump energies. Taking a further step, the same group deposited the RE ion-doped microrods onto an Ag-coated substrate. With the help of surface plasmon resonance, the UC spontaneous emissions of a single microrod were improved by 10 times, and the corresponding excitation threshold of the WGM UC laser was reduced by 50%.⁷⁶ This is an efficient way to reduce the threshold of UC laser from single UCM/NPs. By using the single UCM/NP as the gain medium and a passive microcavity to generate resonance, the UC micro/nano-laser can also be obtained. Shang et al.⁷⁷ coated a single β-NaYF₄:20%Yb³⁺,2%Tm³⁺ nanoparticle on a 5-μm spherical cavity. By pumping with a 980 nm continuous-wave laser beam with a power density of 10 kW/cm⁻², the single nanoparticle lases with a full width at half-maximum (FWHM) as narrow as ∼0.45 nm.

The rich 4f energy levels lead to the cross-relaxation (CR) be often observed between a pair of nearby RE ions, in which one ion at a higher excited energy state transfers its energy to the other ion at a lower excited state or ground state such that both ions become populated in one of their intermediate excited states.¹ The inter-ionic CR allows efficient stimulated emission depletion (STED) to be explored in RE ions and makes single UC nanoparticles suitable for advanced luminescence microscopy applications.⁷⁸ For instance, in Tm³⁺ highly doped NaYF₄, the reduced distance between Tm³⁺ ions leads to intense CR under excitation at 980 nm. The intense CR would induce a photon-avalanche-like effect that the metastable ³H₄ level of Tm³⁺ ions become rapidly populated, which further results in a population inversion on the intermediate metastable ³H₄ level relative to the ³H₆ ground state level. A laser at 808 nm matches well with the energy gap between ³H₄ and ³H₆. Consequently, illumination simultaneously at 808 nm can trigger amplified stimulated emission to depopulate the ³H₄ intermediate level, which suppresses the blue UC emission. Based on this character of Tm³⁺, under the simultaneous excitation of a 980 nm Gaussian excitation beam and an 808 nm donut-shaped depletion beam, Jin et al. and Zhan et al. demonstrated low-power STED in a Yb³⁺/Tm³⁺ co-doped single nanoparticle with a high Tm³⁺ doping concentration, which enables imaging of a single UC particle with nanoscale resolution [Fig. 1(f)].^78,79 The highest resolution reaches 28 nm, that is, 1/36th of the wavelength. Meanwhile, Zhan et al. showed a super-resolution imaging of immunostained cytoskeleton structures of fixed cells using these UC nanoparticles.⁷⁹ Furthermore, under simultaneous excitation at 795 nm and 1145 nm, STED can also be realized in Er³⁺-single doped UC nanoparticles, while the efficiency is much lower than that of Tm³⁺ doped NCs.⁸⁰ These results have strong implications for the applications of single UCM/NPs in sub-diffraction microscopic imaging.

Recently, Jin's group developed an epitaxial growth approach that successfully realized precise doping control in the single-axis of uniform one-dimensional RE ion-doped nanobarcodes [Fig. 1(g)].⁸¹ The nanobarcode is a single UC nanorod that consists of several sections doped, respectively, with different RE ions. More interestingly, the nonlinear UC optical responses and low saturation intensity allow the spread of the emission point, resulting in a much smaller dark area, with the FWHM down to around 29 nm.⁸¹ Based on this result, they employed an annular excitation profile (donut-shaped illumination beam) in a typical confocal microscopy setup and successfully read the optical formation on a single nanobarcode. Each section in a single nanorod display UC emission of four optically orthogonal dimensions, including color, lifetime, excitation wavelength, and power dependence. By using the nanobarcodes with section-dependent UC emission characteristics, multiple codes can be successfully coded and decoded in a single UC nanorod. This finding suggests that precise design and fabrication of heterogeneous nanostructured single UCM/NPs could be possible, which may enable the integration of multiple desirable functionalities in single UCM/NPs.

Developments in materials science and nanophotonic tools offer new opportunities to achieve extremely effective UC behavior characterization of single micro/nanoparticles, leading to a breakthrough insight into UC emission in nanoscale objects. However, despite highly encouraging research results and findings, further effort to push the research to a remarkable depth and breadth is still necessary. First, the major challenge in the investigation of single UCM/NPs lies in the low quantum efficiency, which undermines the detection sensitivity. The improvement of quantum efficiency has always been a challenge in the development of UC materials. Although more efforts have been devoted to improving the UC quantum efficiency (UCQE), the UCQE of small size particles is still low; therefore, successful single UCM/NP detection can only be realized at a relatively high irradiation power density. The higher pumping energy may easily cause side effects and even irreversible damage to the nanoparticle and the substrate. This issue can be partly mitigated by using pulsed laser as the excitation source, while the potential structural damage of single UCM/NPs still cannot be overlooked. Hence, it is urgent to develop high-quality UCM/NPs with high UCQE and to improve the sensitivity by more effective excitation and detection, such as directional excitation and collection of fluorescence signals. On the other hand, up to now, most investigations on single UCM/NPs focused on optical behavior, while a few studies are devoted to examining the effect of the crystal structure. Precise knowledge about structure is extremely important for the optimization and accurate modulation of optical performance. Third, systematic research of single UCM/NPs is expected to be carried out beyond the limits of the current phenomenological exploration. After the addition of external fields, e.g., temperature, pressure, magnetic, and electronic field, we are looking forward to richer photophysical processes in a single UCM/NP.

The authors thank Professor Xiaofeng Liu for revising the paper. This work was financially supported by the Key R&D Program of Guangzhou (No. 202007020003), National Natural Science Foundation of China (NNSFC) (Grant Nos. 5200020611, 62075063, 51772101, and 51872095), the fellowship of the China Postdoctoral Science Foundation (No. 2020M672621), and the Local Innovative and Research Teams Project of Guangdong Pearl River Talents Program (No. 2017BT01X137).

Data sharing is not applicable to this article as no new data were created or analyzed in this study.