All-Copper Nanocluster Based Down-Conversion White Light-Emitting Devices

Abstract

Most of the present-day down-conversion white light-emitting devices (WLEDs) utilize rare-earth elements, which are expensive and facing the problem of shortage in supply. WLEDs based on the combination of orange and blue emitting copper nanoclusters are introduced, which are easy to produce and low in cost. Orange emitting Cu nanoclusters (NCs) are synthesized using glutathione as both the reduction agent and stabilizer, followed by solvent induced aggregation leading to the emission enhancement. Photoluminescence quantum yields (PL QY) of 24% and 43% in solution and solid state are achieved, respectively. Blue emitting Cu nanoclusters are synthesized by reduction of polyvinylpyrrolidone supported Cu(II) ions using ascorbic acid, followed by surface treatment with sodium citrate which improves both the emission intensity and stability of the clusters, resulting in the PL QY of 14% both in solution and solid state. All-copper nanocluster based down-conversion WLEDs are fabricated by integrating powdered orange and blue emitting Cu NC samples on a commercial GaN LED chip providing 370 nm excitation. They show favorable white light characteristics with Commission Internationale de l'Eclairage color coordinates, color rendering index, and correlated color temperature of (0.36, 0.31), 92, and 4163 K, respectively.

Keywords: aggregation‐induced emission enhancement; copper nanoclusters; down‐conversion light‐emitting devices; photoluminescence; white light.

Scheme 1

Schematics of the formation and treatment of orange and blue emitting Cu NCs, and their combination as phosphors in a down‐conversion WLED.

Figure 1

Structural and optical characterization of orange emitting Cu NCs: a) TEM image of agglomerated clusters, inset shows an enlarged view of a few quasi‐spherical agglometartes; b) XPS spectrum of Cu 2p electrons; c) FTIR spectrum of Cu NCs (red line) compared with those of GSH (black line); d) absorption spectrum (green line), PLE spectrum (black line) at detection wavelength of 600 nm, and PL spectrum (red line) at the excitation wavelength of 365 nm.

Figure 2

a) PL spectra (excitation wavelength 365 nm) and b) PL decay profiles (excitation wavelength 320 nm; recorded at the corresponding emission maximum) of the orange emitting Cu NCs in the water/ethanol mixture with increasing volume fraction of ethanol, f _ethanol (%). Inset in (a) shows the photographs of the respective solutions under UV light, demonstrating increasing emission intensity.

Figure 3

Structural and optical characterization of blue emitting Cu NCs: a) TEM image; b) XPS spectrum of Cu 2p electrons; c) absorption (solid lines) and PL (dashed lines, excitation wavelength 380 nm) spectra of the samples before (red) and after (black) citrate treatment; d) PL decay profile (excitation at 320 nm, recorded at emission peak of 420 nm); e) absorption (solid lines) and PL (dashed line) spectra taken on the citrate treated sample after different storage time (blue line – 0 d, red line – one week; black line – one month); f) absorption (solid lines) and PL (dashed line) taken on the sample without citrate treatment after different storage time (blue line – 0 d, red line – one week).

Figure 4

Optical characterization of NC powders (shown as insets under UV illumination). PLE spectra of blue Cu NCs (solid blue line, detection wavelength 460 nm) and orange Cu NCs (solid orange line, detection wavelength 580 nm), as well as PL spectra of blue Cu NCs (dotted blue line) and orange Cu NCs (dotted orange line), both excited at 365 nm.

Figure 5

Emission spectra of monochrome down‐conversion LEDs fabricated by using a) blue emitting Cu NCs and b) orange emitting Cu NCs. c) An emission spectrum of the WLED fabricated by combination of these two kinds of Cu NCs. Insets in (a), (b), and (c) provide photographs of operating blue, orange, and white LEDs, while d) shows CIE coordinates of three respective LEDs.