Trap-controlled mechanoluminescent materials

Abstract

Mechanoluminescence (ML) is generated during exposures of certain materials to mechanical stimuli. Many solid materials produce ML during their fracturing, however, the irreversibility of fracto-induced ML limits the practical applications of these materials. In 1999, Chao-Nan Xu discovered an intense and reproducible ML from trap-controlled materials, including ZnS:Mn 2+ and SrAl 2 O 4 :Eu 2+ , and introduced the principles and applications of hybrid inorganic/organic mechanoluminescent (ML) composites, and related sensors to visualize stress/strain in target structures. This discovery has triggered intense research interest in trap-controlled ML materials and composites over the past 2 decades. Notable achievements of this research include the development of trap-controlled materials that exhibit bright ML emission from the ultraviolet to the near infra-red, and multiscale mechano-optical sensitivities. This research has also increased our understanding of the mechanisms of ML phenomena, enabling the rational design of trap-controlled ML materials. Practical applications of ML are also being driven by the discovery that ML composites can serve as "mechano-optical sensitive skin" for structural health diagnosis, stress sensors for biomechanics, and mechanically-activated light sources. This review focuses on the design, synthesis, characterization, optimization and application of trap-controlled ML materials, and concludes with discussions on future directions of ML research and specific challenges to improve ML materials for real-world applications.

Short-Term Non-Decaying Mechanoluminescence in Li2MgGeO4:Mn2+

“…Characterization of emission spectra of AG shows that the AG of Li2MgGeO4:Mn 2+ originates from the 4 T1( 4 G)-6 A1( 6 S) transition of Mn 2+ coordinated tetrahedrally ( Figure 1d) [31]. These trap levels may come from defects generated during the process of material preparation, including lithium vacancies, oxygen vacancies, and doping traps [18]. ML characterization demonstrates that Li2MgGeO4:Mn 2+ can respond to non-destructive mechanical stimuli including friction and compression by ML (Figure 2a).…”

“…Mechanoluminescent (ML) materials that can quantitatively convert mechanical stimuli into light emission have drawn great attention due to potential applications ranging from stress distribution visualization [1,2] and structural health diagnosis [3][4][5] to light sources [6][7][8][9][10][11][12][13] and anti-counterfeiting [14][15][16]. Dozens of inorganic ML materials with reproducible mechanoluminescence (ML) have been developed during the past two decades [17,18]. Based on whether ML requires light irradiation to recover, the reproducible ML materials can be divided into two types.…”

“…Based on whether ML requires light irradiation to recover, the reproducible ML materials can be divided into two types. One type that does not require irradiation is called self-reproducible ML materials [18]. For example, ZnS:Mn 2+ and ZnS:Cu + exhibited robust and non-decaying ML over many thousands of cycles of a mechanical stimulus [1,19,20].…”

“…However, self-reproducible ML materials are mainly limited to transition metal-doped ZnS series and the underlying mechanism of self-reproducibility remains unclear. Another type that requires irradiation is trap-controlled ML materials [18]. Most developed reproducible ML materials are of this type, such as SrAl 2 O 4 :Eu 2+ ,Dy 3+ [21], CaYAl 3 O 7 :Ce 3+ [22], BaSi 2 O 2 N 2 :Eu 2+ [23], Sr 3 Sn 2 O 7 :Sm 3+ [24], and LiNbO 3 :Pr 3+ [25], as well as CaZr(PO 4 ) 2 :Eu 2+ [26], (Ca,Sr) 2 Nb 2 O 7 :Pr 3+ [27,28], Ca 3 Ti 2 O 7 :Pr 3+ [29], NaNbO 3 :Pr 3+ ,Er 3+ [14], and La 1.95 Ti 2 O 7 :Pr 3+ [15], developed by our lab.…”

A flexible topo-optical sensing technology with ultra-high contrast

“…By utilizing soft materials, recent efforts have explored the flexible optical technology with extra controllability and ondemand color changing such as triboelectric-photonic 12,13 , piezoelectroluminescent 14 , piezo-photonics [15][16][17] , mechano-responsive luminescence (MRL) and mechanochromism 18 . Among those approaches, MRL, a tunable and switchable luminescence (or chromism) in response to mechanical stimulus 19,20 , have attracted considerable interests for their potentials in sensing/micro-devices 21 , data storage 22,23 , flexible display 24,25 , security pattern/ inks 26 , etc. However, the optical performance has been discounted by aggregation-caused quenching (ACQ) 27,28 , thus limit the further applications for MRL materials.…”