Single-Particle Studies Reveal a Nanoscale Mechanism for Elastic, Bright, and Repeatable ZnS:Mn Mechanoluminescence in a Low-Pressure Regime

Abstract

Mechanoluminescent materials, which emit light in response to elastic deformation, are demanded for use as in situ stress sensors. ZnS doped with Mn is known to exhibit one of the lowest reported thresholds for appearance of mechanoluminescence, with repeatable light emission under contact pressure <10 MPa. The physical basis for such behavior remains as yet unclear. Here, reliable microscopic detection of mechanoluminescence of single ZnS:Mn microparticles, in combination with nanoscale structural characterization, provides evidence that the mechanoluminescent properties of these particles result from interplay between a non-centrosymmetric crystal lattice and its defects, viz., dislocations and stacking faults. Statistical analysis of the distributions of mechanoluminescence energy release trajectories reveals two distinct mechanisms of excitation: one attributable to a piezo-phototronic effect and the other due to dislocation motion. At pressures below 8.1 MPa, both mechanisms contribute to mechanoluminescent output, with a dominant contribution from the piezo-phototronic mechanism. In contrast, above 8.1 MPa, dislocation motion is the primary excitation source. For the piezo-phototronic mechanism, we propose a specific model that accounts for elastic ZnS:Mn mechanoluminescence under very low pressure. The charged interfaces in stacking faults lead to the presence of filled traps, which otherwise would be empty in the absence of the built-in electric field. Upon application of external stress, local enhancement of the piezoelectric field at the stacking faults' interfaces facilitates release of the trapped carriers and subsequent luminescence. This field enhancement explains how <10 MPa pressure produces thousands of photons.

Keywords: built-in electric fields; elastic mechanoluminescence; microplasticity; single-particle luminescence; stacking faults; traps.

Figure 1:

a) SEM image of ZnS:Mn microparticles, the red line outlines the hexagonal shape of a single MP, the white rectangle outlines the region enlarged in the inset. Inset: white arrows show stacking faults (TEM characterization in Figure 5). b) Experimental setup for real-time ML detection (see text; Methods). c) Time-resolved mechanoluminescence of a single ZnS:Mn microparticle plotted along with exciting pressure pulse; a MP diameter was 4.6 μm; the MP was embedded in PDMS that covered the tip of the probe; ML images were collected with exposure time of 50 msec (red circles indicate the start of every exposure cycle), pressure was detected every 8 msec (black bars); the range of ML registration wavelengths was λ_reg= 570 – 620 nm, strain rate was 0.125 μm/msec; ML curve is shifted forward for 65 msec to correct for the time lag in detection of changes in pressure (see Note S1); inset shows 2D images of the ML intensity distributions at the three time points numbered in the main figure. d) Distribution of threshold pressures for ML appearance for 44 single MPs at pressures ranging from 0.23 – 47.15 MPa. Inset: threshold pressure plotted against MP diameter (the diameter is estimated from PL images of the particles).

Figure 2:

Example 1 (a-e) and 2 (f-h) include data for two different single ZnS:Mn microparticles. a) Time-resolved mechanoluminescence of a single MP excited by cyclic compressive pressure of 5.6 MPa; exposure time of ML registration was 50 msec, pressure was detected every 8 msec; the range of ML registration wavelengths was λ_reg= 570 – 620 nm, strain rate was 0.125 μm/msec; ML curve is shifted forward for 295 msec to correct for the time lag in detection of changes in pressure (see Note S1). b) Zooming in on the last ten ML pulses shown in a). c) ML emission rate Γ_ML calculated for each ML pulse within series excited by cyclic pressure of 5.6 MPa (red dots), 6.4 MPa (blue dots), 24 MPa (green dots) and plotted versus number of pressure cycle scaled by pulse width w; filled areas under each Γ_ML curve show fitted Gaussian functions corresponding to Pz- (solid filling) and μD-D-component (lined filling) of Γ_ML. d) Total photon counts emitted by the MP within each series shown in c) (black dots), also plotted separately for Pz- (solid filling) and μD-D-component (lined filling) of Γ_ML; in addition, black dots are used to show the series with zero total photon counts. e) 2D PL images of the MP for data shown in c) taken before (1) and between pressure application series (4,6,8); sum projection of 2D ML images taken during the periods shown with curly brackets (2,3,5,7); yellow circles show ML roi. f) Same as c) for different MP, red, blue, green, and violet dots correspond to excitation pressure of 7.4, 13.6, 20.5, 26.1 MPa, respectively. g) Same as d) for data shown in f). h) Same as e) for data shown in f), along with time-resolved distribution for the second peak of the third series (4).

Figure 3:

Empirical distributions (as complementary cumulative density functions (CDFs) ) of energy in the ML pulses series shown in Figure 2c excited by cyclic compressive pressure of 5.6 MPa (a-c) and 24 MPa (c), along with the best fitting power-law distribution (dashed line) and stretched-exponential distribution (dotted line); (a) includes all data points for the excitation pressure of 5.6 MPa, whereas (b) shows only the last 63 data points (Pz-component) of 5.6-MPa dataset; (c) represents the distributions for combined μD-D-components for the excitation pressure of 5.6 MPa (the first eight data points) and 24 MPa (all data points).

Figure 4:

a) Time-resolved mechanoluminescence of a single MP excited by a single pulse of pressure; exposure time of ML registration and temporal resolution of pressure detection were 0.633 msec; red dots mark the first and the last frames of the ML pulse shown in b). b) Time lapse images of ML peak revealing striation. c) Sum projection of all frames for the ML flash shown in b). d) PL image of the particle from b) and c) taken at the plane of ML registration; red lines outline outer and inner grain boundaries inside the particle as revealed by thresholding PL image of the particle (12.55% at the middle range of intensity). e) Merged ML (red) and middle plane PL (blue) images of the particle from b)-d) showing that stripes in ML and PL patterns do not overlap. f) 3D isosurface representation of grain boundary (one plane shown in d), outer luminescent layer is removed for clarity, red contour line highlights the MP z plane shown in panels b)-e). PL images are taken with the excitation wavelength λ_ex= 365 nm, the registration wavelength λ_reg= 570 – 620 nm is the same for both ML and PL registration.

Figure 5:

a) Bright field TEM image of a FIB lamella of a ZnS:Mn microparticle (not the same one imaged optically) viewed down the $[11\overline{2}0]$ zone axis with inset electron diffraction pattern (top) and simulated diffraction pattern (bottom). b) Zoomed in bright field TEM image from the upper part of the particle roughly from the region in the red box in a) with inset electron diffraction pattern. c) High resolution TEM image of the stacking disorder with a d) FFT from the whole image (we note all FFTs were carefully processed with a hanning window to avoid image edge artifacts), e) a pristine wurtzite region, and f) a faulted region. In total, the particle has a basal plane stacking fault density of ~11 stacking faults per micron along the c-axis. However, this distribution is not uniform, and in the highly faulted areas, the density can be as high as 40 faults/μm.

Figure 6:

a) shows SEM image of the MP after FIB treatment, red dashed rectangular shows one of the areas scanned by AFM. b) AFM image of microparticle interior exposed by FIB treatment. Topology is shown as 3D profile, which is color-coded to visualize KPFM potential. c) Profiles of height (blue) and KPFM potential (red) along the white line shown in b). d) Distribution of potential barriers height calculated from KPFM potential data for different areas of the sample.

Figure 7:

Energy band diagram illustrating the model of mechanically induced mechanoluminescence in faulted ZnS:Mn. Panels represent a WZ/ZB/WZ sequence that comprises the structural unit of the model a) in absence of external stress, b) compressed, c) dilated, and d) after the multiple emission cycles. Red pluses and blue minuses represent polarization charges created by electric fields and localized states, free electrons and holes are shown by blue and empty red circles, respectively. For the WZ phase, we show 2H modification with maximum hexagonality; however, a distribution of 6H, 4H and 2H modifications can exist in the crystals. The ZB phase has only 3C modification, with the 2H/3C interface producing the highest potential barrier. Grey dotted lines on panels (b-d) shows bands before pressure application. Other notations are defined in the text.

Scheme 1:

ML mechanisms operating at different pressure range with corresponding properties defined in text; Γ_ML is ML emission rate, w_ML is width of ML pulse, w_P is width of pressure pulse, *provisional limit of microplastic regime defined by the lowest pressure at which μD-component is observed in our experiments.