Enhanced Red Upconversion in Single β‑NaYF4:Yb/Er Microcrystals Doped with Mn²⁺: Tunable Multicolor Emission and Energy‑Transfer Dynamics

Abstract

Adding Mn²⁺ to Yb/Er‑co‑doped β‑NaYF4 nanomaterials suppresses the green (545 nm) band and boosts the red (650 nm) upconversion (UC) output, enabling single‑red‑band emission for bio‑imaging and drug delivery. Here, we re‑examine a single Mn²⁺‑doped β‑NaYF4:Yb/Er microcrystal synthesized by a straightforward one‑pot hydrothermal route. Under 980 nm CW excitation, the microrod’s color shifts from green to red as Mn²⁺ concentration rises from 0 % to 30 % (mol). At high pump powers, a new 560 nm band (²H₉/₂ → ⁴I₁₃/₂) emerges and surpasses the conventional green (545 nm) line. Consequently, we define two red‑to‑green (R/G) ratios: the traditional 650/545 nm and the new 650/560 nm. Both ratios track Mn²⁺ doping at low excitation, but diverge under intense illumination. Energy transfer between Mn²⁺ and Er³⁺ governs the R/G modulation, offering a versatile multicolor platform for display and micro‑optoelectronics.

Introduction

Lanthanide‑doped upconverting nanomaterials have attracted intense research interest due to their unique spectroscopic fingerprints. β‑NaYF4 doped with Yb³⁺/Er³⁺ (or Tm³⁺/Ho³⁺) is a benchmark system, delivering efficient NIR‑to‑visible UC suitable for color displays, super‑resolution imaging, security inks, laser media, and biological probes. The rich 4fⁿ electronic manifold of lanthanides yields multiple emission bands, which, while useful, can compromise quantitative imaging and limit sensitivity. Consequently, strategies to suppress multi‑band output and achieve single‑band UC have emerged. One effective approach is to introduce transition metals such as Mn²⁺, which enhance the 650 nm emission via strong energy‑transfer (ET) to Er³⁺, yielding single‑red UC.

Previous work has explored Mn²⁺‑doped Yb/Er nanocrystals for biomedical imaging, sensing, and biomarker detection. However, microcrystals—owing to their superior crystallinity and higher luminescent yield—are better suited for micro‑optoelectronic devices, volumetric displays, and microlasers. Unfortunately, most characterizations have relied on powder or solution samples, where thermal loading and inter‑particle interactions can distort UC behavior. Studying UC from isolated microcrystals therefore offers a clearer understanding of intrinsic properties and paves the way for device integration.

When excited at 980 nm, Yb/Er co‑doped materials typically emit red (650 nm), green (525 nm/545 nm), and a weaker blue (410 nm) UC. In prior studies, a novel 560 nm band (²H₉/₂ → ⁴I₁₃/₂) appeared under saturated excitation, eventually outshining the traditional green line. In tri‑doped Yb/Er/Mn systems, this transition also acts as an ET channel to populate the Mn²⁺ ⁴T₁ level, a mechanism not yet reported. Consequently, the suppression of the new green band and the tunability of R/G ratios remain largely unexplored.

In this work, we synthesize Mn²⁺‑doped β‑NaYF4:Yb/Er microcrystals via a simple one‑pot hydrothermal method and perform single‑particle UC spectroscopy using a high‑NA inverted microscope. We demonstrate continuous color tuning from green to red as Mn²⁺ content increases, analyze both traditional and new R/G ratios across excitation regimes, and elucidate the underlying ET pathways.

Methods

Materials

All reagents (Y₂O₃, Yb₂O₃, Er₂O₃, MnCl₂·4H₂O, HNO₃, EDTA‑2Na, NaOH, NH₄F) were purchased from Aladdin (China) and used without further purification.

Synthesis of β‑NaYF4 Microcrystals

β‑NaYF4:Yb/Er/Mn (20/2/x mol %) microcrystals were prepared by dissolving the lanthanide oxides in dilute nitric acid, followed by complexation with EDTA‑2Na and NaOH. MnCl₂·4H₂O was added to achieve the desired Mn²⁺ molar fraction while keeping total lanthanide content constant (1 mmol). The mixture was stirred for 1.5 h, transferred to a 50 mL Teflon‑lined autoclave, and heated at 200 °C for 40 h. After centrifugation, the precipitate was washed with deionized water and ethanol, then dried at 40 °C for 12 h.

Physical Characterization

XRD was recorded on a Rigaku diffractometer (Cu Kα, 40 kV, 200 mA). Morphology was examined by SEM (Hitachi S4800).

Photoluminescence Measurements

UC was excited by a 980 nm CW laser focused through a 100× objective (NA = 1.4) onto a single microrod (≈ 2 µm spot). Emission was collected by the same objective, dispersed on an Andor SR‑500I spectrometer, and recorded with a DU970N CCD. Color images were captured with a Nikon DS‑Ri2 camera. Lifetime measurements employed a 20 ns pulsed laser (10 Hz) and a 1 GHz oscilloscope.

Results and Discussion

SEM images reveal uniform hexagonal microrods (~ 3.5 µm diameter, ~ 13 µm length). Increasing Mn²⁺ to 30 % slightly reduces length to 10 µm, but crystal phase remains hexagonal as confirmed by XRD. No secondary phases appear, indicating Mn²⁺ incorporation does not disturb the host lattice. EDS confirms the presence of Na, F, Y, Yb, Er, and Mn (at 30 % doping).

UC spectra of a single β‑NaYF4:Yb/Er microrod (0 % Mn) display the expected 410 nm (²H₉/₂ → ⁴I₁₅/₂), 525/545 nm (⁴S₃/₂/²H₁₁/₂ → ⁴I₁₅/₂), and 650 nm (⁴F₉/₂ → ⁴I₁₅/₂) bands. With Mn²⁺, the 650 nm band strengthens, eventually dominating the spectrum at 30 % Mn, shifting the observed color from green to red. CIE chromaticity coordinates trace a continuous trajectory from green to red as Mn²⁺ increases.

At higher excitation (≈ 96 kW cm⁻²), additional UC bands appear: 382, 457, 472, 506, 560, and 618 nm, arising from higher‑lying Er³⁺ transitions. The 560 nm band, usually non‑radiative, becomes prominent and can surpass the 545 nm line under strong pumping.

By monitoring a 10 % Mn sample across excitation powers, we observe that the 560 nm intensity rises faster than the 545 nm line, eventually exceeding it at ≈ 32 kW cm⁻². The red (650 nm) emission continues to grow, but the ratio 650/560 nm decreases at high power, reflecting competition between radiative 560 nm emission and ET to Mn²⁺. Lifetime studies show the 650 nm decay is longest, confirming efficient population of the ⁴F₉/₂ level via ET from Mn²⁺ back‑transfer (ET₃). Lifetimes of the green bands shorten with Mn²⁺, indicating accelerated ET to Mn²⁺ (ET₁ and ET₂). Efficiency calculations yield η₁ ≈ 34 % and η₂ ≈ 41 % for 30 % Mn, confirming the dominant role of ET₂ in enhancing red UC.

Thus, the tunable color arises from a cascade: Er³⁺ energy is transferred to Mn²⁺ ⁴T₁ (ET₁/₂), then back‑transferred (ET₃) to populate the ⁴F₉/₂ state, boosting red emission while simultaneously quenching the green bands. The new 560 nm transition competes with ET₁, explaining its intensity evolution with pump power.

Conclusion

We have demonstrated continuous green‑to‑red UC tuning in a single Mn²⁺‑doped β‑NaYF4:Yb/Er microcrystal by adjusting Mn²⁺ content and excitation intensity. The emergence of a 560 nm band under high power reshapes the R/G ratio landscape. These findings provide a deeper understanding of multicolor UC control and open avenues for high‑efficiency micro‑display and optoelectronic devices.

Abbreviations

CCD

Charge‑coupled device

CW

Continuous wave

DI

Deionized

ET

Energy transfer

Mn²⁺

Manganese ions

NIR

Near‑infrared

R/G

Red‑to‑green ratio

SEM

Scanning electron microscopy

UC

Upconversion

XRD

X‑ray diffraction