Fabrication of a Functionally Graded Copper-Zinc Sulfide Phosphor

Functionally graded materials (FGMs) are compositionally gradient materials. They can achieve the controlled distribution of the desired characteristics within the same bulk material. We describe a functionally graded (FG) metal-phosphor adapting the concept of the FGM; copper (Cu) is selected as a metal and Cu- and Cl-doped ZnS (ZnS:Cu,Cl) is selected as a phosphor and FG [Cu]-[ZnS:Cu,Cl] is fabricated by a very simple powder process. The FG [Cu]-[ZnS:Cu,Cl] reveals a dual-structured functional material composed of dense Cu and porous ZnS:Cu,Cl, which is completely combined through six graded mediating layers. The photoluminescence (PL) of FG [Cu]-[ZnS:Cu,Cl] is insensitive to temperature change. FG [Cu]-[ZnS:Cu,Cl] also exhibits diode characteristics and photo reactivity for 365 nm -UV light. Our FG metal-phosphor concept can pave the way to simplified manufacturing of low-cost and can be applied to various electronic devices.

(ZnS:Cu,Cl) as metal and phosphor materials, respectively. Cu and ZnS:Cu,Cl were successfully combined by adapting the concept of the FGM. The fabricated FG [Cu]-[ZnS:Cu,Cl] showed that one side had the intrinsic properties of an electrode and other side had the intrinsic properties of a phosphor within the same bulk material. To the best of our knowledge, this is the first structure that combines metal and phosphor as an adaptation of the FGM concept. In this paper, we investigate the luminescent properties and reactivity for ultraviolet (UV) -light as well as the morphology of the FG [Cu]-[ZnS:Cu,Cl].

Results
Functionally graded (FG) metal (Cu)-phosphor (ZnS:Cu,Cl) was fabricated by using a very simple powder process. Our FG [Cu]-[ZnS:Cu,Cl] revealed dual-structured functional materials composed of dense Cu and porous ZnS:Cu,Cl which were completely combined through six graded mediating layers and bluish-green light was emitted under a 365-nm UV lamp in the layer of ZnS:Cu,Cl (Figs 1 and 2). In the photoluminescence (PL) study, our FG [Cu]-[ZnS:Cu,Cl] exhibited stability upon temperature change because the Cu intimately combined with the ZnS:Cu,Cl. Cu acts as a heat sink in dual-structured FG [Cu]-[ZnS:Cu,Cl] (Fig. 3). In addition, the FG [Cu]-[ZnS:Cu,Cl] exhibited diode characteristics and photo reactivity for 365 nm-UV light (Fig. 4). Figure 1 Figure 1(b) shows a cross-sectional photograph of the FG [Cu]-[ZnS:Cu,Cl] cut by a diamond saw. The cross section shows a clear surface that was not delaminated or cracked between layers while cutting. This is evidence that the Cu and ZnS:Cu,Cl layers were completely combined. The Cu and ZnS:Cu,Cl layers can be obviously identified when 365 nm-ultraviolet (UV) light was irradiated on the cross-section of the specimen [ Fig. 1(c), movies S1 and S2]. We confirmed the cross-sectional morphology of the FG [Cu]-[ZnS:Cu,Cl] using scanning electron microscopy (SEM). Figure 1 Figs  S2 and S3 of SI). Additionally, we can confirm that the density of the Cu layer is higher than that of the ZnS:Cu,Cl layer. We expect that this dual structure of porous-dense material has an advantage in sensor applications 25,26 . In our   case, the ZnS:Cu,Cl layer functions as the active layer detecting a change in the environment (absorption of photons or gas [12][13][14][15][16][17], the porous structure of the ZnS:Cu,Cl expose more active sites to the environment than a dense structure would, and the Cu layer functions as an electrode that transfers electrons captured by the active sites of ZnS:Cu,Cl; the dense structure of the Cu provides more electron mobility than a porous structure would.

Discussion
From the results of the EDS line scan and elemental mapping [ Fig. 1(e)], we confirmed that Zn and S gradually decreased while Cu gradually increased moving towards the Cu layer. Additionally, a small Cu signal was detected in the ZnS:Cu,Cl layer, while small Zn and S signals were detected in the Cu layer, due to the inter-diffusion of Zn, S and Cu during the SPS process.  Cu,Cl] were well matched with the primary powders, and minor phase was slightly observed in graded layer; the XRD pattern of the graded layer revealed a bi-phase mixture of ZnS and Cu including slight third phase (Fig. S4). Interestingly, the XRD patterns of the ZnS in the ZnS:Cu,Cl and the graded layers were highly oriented to the (111)-crystal plane compared with the patterns of the ZnS:Cu,Cl powder. This is due to the effect of uniaxial pressure, where randomly oriented grains are re-arranged as a result of the pressure applied during the SPS process, thus graining a preferred orientation after SPS 27 . Figure S5 Fig. 1(e)]. In addition, the reflection-peak position of ZnS in the ZnS:Cu,Cl layer shifted to a slightly larger angle than that of the graded layer [ Fig. S5(a)]. We assume that this small shift is caused by sublimation of volatile S in the ZnS crystals during the SPS process, resulting in structural defects in ZnS caused by S vacancies that can induce lattice distortion.  Fig. S5. We assume that excited electrons are trapped at V S sites and Cl S sites, and that these trapped electrons recombine at Cu Zn sites.
Additionally, we investigated the PL properties of FG [Cu]-[ZnS:Cu,Cl] and ZnS:Cu,Cl powder as a function of temperature (Fig. 3). Table 1 presents the PL-peak position of FG [Cu]-[ZnS:Cu,Cl] and ZnS:Cu,Cl powder attained from the PL spectra in Fig. 3. In phosphors, a change of the PL -peak position with temperature is caused by electron-phonon interaction 23 (Fig. 4). These curves reveal asymmetric and nonlinear behaviour that resembles a Schottky diode. We cautiously assume that this behaviour is due to the graded layer of FG [Cu]-[ZnS:Cu,Cl] having graded electrical conductivity according to the Cu content. We speculate that the graded layer can affect the electron mobility under forward and reverse bias; a detailed study of the I-V characteristics is currently in progress. In any case, when the FG [Cu]-[ZnS:Cu,Cl] was irradiated by 365 nm -UV light, the current increased from 1.02 mA (dark condition) to 1.40 mA at an applied bias of 5 V [ Fig. 4(a)] and the current increased from 10.34 μA (dark condition) to 20.05 μA at an applied bias of 0.5 V [ Fig. 4(b)]. From these results, we conclude that our FG [Cu]-[ZnS:Cu,Cl] demonstrates reactivity for UV-light and that more quantitative study is needed to understand its performance in terms of photosensitivity, efficiency, and wavelength-dependent responsivity.
In summary, we successfully fabricated a novel functionally graded (FG) metal (Cu)-phosphor (ZnS:Cu,Cl) using a very simple powder process.

Methods
Fabrication of functionally graded (FG) [Cu]-[ZnS:Cu,Cl] was carried out using the following simple powder process.
High-purity commercial Cu powder (dendrite, 99.99% ) and commercial Cu-and Cl-doped ZnS (ZnS:Cu,Cl, cubic, ~50 μm) were used as a raw materials. Mixtures of Cu and ZnS:Cu,Cl powders containing 5, 10, 20, 30, 50, 70 vol.% Cu in ZnS:Cu,Cl were prepared using a simple ball milling process for 30 min at 200 rpm in air. The pure Cu and ZnS:Cu,Cl powders were not ball-milled. Pure Cu, mixtures of Cu-ZnS:Cu,Cl, and pure ZnS:Cu,Cl powders were stacked layer by layer into a graphite mould with a diameter of 15 mm; 0.2 g of powder was used for each layer. Then the stacked powders were sintered using a customized spark plasma sintering (SPS) system (Fuji Electronic Industrials Co., Ltd., SPS-321Lx, Japan). The SPS was carried out at 900 °C (heating rate; 100 °C/min) for 5 min of 50 MPa of pressure. The fabricated FG [Cu]-[ZnS:Cu,Cl] was shaped as a round disk with a diameter of 15 mm and a thickness of approximately 1.7 mm, as shown in Fig. 1.
The morphology and structures of fabricated FG [Cu]-[ZnS:Cu,Cl] were analysed by using a scanning electron microscope (SEM, Tescan, Vega, Czech) equipped with an energy-dispersive spectrometer (EDS, Horiba, Emax, Japan) and an X-ray diffractometer (XRD, Rigaku, Ultima, Japan). The XRD operated at 40 kV and 40 mA with Cu Kα radiation. The photoluminescence (PL) spectra of fabricated FG [Cu]-[ZnS:Cu,Cl] and ZnS:Cu,Cl powder were obtained with a Darsa pro-5200 system (PSI, Korea) equipped with a temperature sensor using an excitation wavelength of 365 nm from a xenon lamp. In the case of ZnS:Cu,Cl powder, the powder was pressed into a disk with a diameter of 15 mm and a thickness of approximately 1.7 mm before the PL measurement. Current-voltage (I-V) curves of the FG [Cu]-[ZnS:Cu,Cl] were obtained using a Keithley 2400, and a 365 nm-UV source (UVItec Ltd, LF-204.LS, 4W, UK) was used for irradiation at distance of 10 cm between source and sample.