SHANG Mengmeng，SUN Yixin

College of Materials Science and Engineering， Shandong University， Jinan 250061， China

Abstract

Significance In recent years， near-infrared （NIR） fluorescent materials have gained significant attention due to their wide applications in food safety， medical diagnosis， modern agriculture， and environmental protection. The emission bandwidth of NIR fluorescent materials is crucial for the sensitivity and detection range of NIR spectroscopy. Novel NIR fluorescent materials activated by rare earth ions （Nd³⁺ 、 Dy³⁺ 、 Er³⁺ 、 Yb³⁺ ） or transition metal ions （Cr³⁺ 、 Ni²⁺ 、 Mn^2+/4+ ， and Fe³⁺ ） have been widely reported. However，trivalent rare earth ions have narrow emission bandwidths and low absorption efficiency. Although transition metal ions with d-d transitions exhibit a wider emission range， their emission half-peak width and excitation-emission wave⁃length mismatch in the NIR range are suboptimal. Cr ions are ideal NIR luminescence centers with broad absorption in the UV-visible spectrum. Understanding the valence states and lattice sites of Cr ions is essential for developing high-performance Cr ion-activated ultra-broadband NIR fluorescent materials， which are significant for NIR spectroscopy applications.

Progress This review discusses the two common valence states （+3 and +4）and three luminescent centers of Cr ions in NIR fluorescent materials： hexacoordinated Cr³⁺，tetracoordinated Cr³⁺， and tetracoordinated Cr⁴⁺. Hexacoordinated Cr³⁺ occupies octahedral lattice sites， and its luminescence varies with crystal field strength， displaying either sharp peaks or broad emissions.In spinel compounds， elements such as Mg and Al influence site occupancy， causing Cr³⁺ ions to occupy tetrahedral lattice sites， resulting in emission peaks that tend to appear at relatively longer wavelengths. Cr⁴⁺ typically emits in the 1 000~1 400 nm range， with peak wavelengths exceeding 1 200 nm. The review summarizes two design methods for Cr ion-doped ultra-broadband NIR fluorescent materials： co-doping Cr³⁺ with other ions such as rare earth （RE³⁺ ） and transition （Ni²⁺ ） ions，and selecting matrix materials with different cation lattice sites to achieve varying Cr ion valence states or having Cr ³⁺ occupy multiple lattice sites. The emission mechanisms of these methods are compared， highlighting their advantages and disadvantages. When co-doping Cr ions with RE³⁺ ions， Cr ions can act as sensitizers to broaden the infrared emission range， although this may reduce Cr ion emission intensity.

Conclusions and Prospects Selecting matrix materials with weak crystal field environments and regulating lattice sites are effective strategies for obtaining efficient Cr-doped ultra-broadband NIR fluorescent materials. By occupying different lattice sites，Cr ions can achieve a wider NIR emission range and higher concentration quenching values due to the increased total Cr ion content. These regulatory methods are also flexible， providing various ways to optimize material performance.

Keywords：Cr ion doping； near-infrared emission； luminescence mechanism