Photoconductivity and Performance of Mn 2+ and Ce 3+ Doped ZnS Quantum Dot Detectors

Mn 2+ and Ce 3+ Doped ZnS nanocrystals were prepared by a simple microwave irradiation method under mild condition. The starting materials for the synthesis of Mn 2+ and Ce 3+ Doped ZnS P nanocrystals were zinc acetate as zinc source, thioacetamide as a sulfur source, manganese chloride and Cerium chloride as manganese and cerium sources respectively (R & M Chemical) and ethylene glycol as a solvent. All chemicals were analytical grade products and used without further purification. The nanocrystals of Mn 2+ and Ce 3+ Doped ZnS P with cubic structure were characterized by X-ray powder diffraction (XRD), the morphology of the film is seen by field effect scanning electron microscopy (FESEM). The composition of the samples is analyzed by EDS. The spectral response of Mn 2+ and Ce 3+ Doped ZnS nanocrystals was studied. The values of responsively, specific detectivity and quantum efficiency for Ce 3+ Doped ZnS are higher than that for Mn 2+ Doped ZnS.

is defined as a cluster that contains from a few hundred to tens of thousands of atoms, ranging anywhere from a couple of nanometers to tens of nanometers in size.Nanocrystals are very attractive and interesting because they have unique properties that set them apart from typical bulk materials.These unique properties result because nanocrystals are larger than molecules, but much smaller than bulk materials.As a result, nanocrystals show properties that are somewhere in between the properties of discrete molecules and bulk materials [1].When the size or shape of nanocrystals is changed, their properties can also be altered accordingly, enabling scientists to tune them for specific purposes.In addition, it is easy to grow flawless, perfect nanocrystals since their length of scale is so small that there is barely any time to introduce defects [2,3].Because of these reasons, much investigation is dedicated to the study of nanocrystals and how their special properties can be applied to practical applications.
Photoconductivity (PC) is defined as electrical conductivity resulting from photoinduced electron excitations in which light is absorbed.In semiconductors, photoconductivity arises due to interaction of photons with bound electrons of lattice atoms that leads to photo-generation of electron-hole pairs after absorption of photons which increases carrier density and conductivity of material [4,5].Extensive study of photoconductivity has been made in nanoparticles, thin film, nanorods, nanowires and mixed lattice [6][7][8] for different parameters.
Wide-bandgap II-VI compounds have been applied to optoelectronic devices, especially light emitting devices in the short-wavelength region of visible light.Here our focus is on ZnS semiconductor which is studied in the present study.Zinc sulfide (ZnS) is one of the most typical and important crystalline materials for both application and research [1,9].Research studies are carried out on doped II-VI semiconductor nanomaterials to enhance their light emission properties and thereby making them good candidate for optoelectronic applications such as displays, sensors and lasers.The doping of ZnS with transition metals such as Mn, and rare earth such as Ce is interesting to researchers because of the effect of dopant on the photoluminescence and photoreactivity properties of the semiconductor [10,11,12].

EXPERIMENTAL
The starting materials for the synthesis of Mn and Ce doped ZnS NCs were zinc acetate as zinc source, thioacetamide as a sulfur source, manganese chloride and Cerium chloride as manganese and cerium sources respectively and ethylene glycol mixed with distilled water as a solvent.In a typical synthesis, 5 mM of zinc source and 0.05mM of manganese or cerium source were added with appropriate concentrations of Manganese or cerium (1wt.%) in a glass beaker of 80 mL containing 20 mL of ethylene glycol and 60ml distilled water as solvent .The solution was stirred for 1hour at 70 oP C, and then 6 mM of sulfur source dissolved in 20ml was added by drop wise and stirred for 30 min.The beaker was placed in a high power microwave oven (1100 W) operated using a pulse regime with 20% power for 25 min irradiation time.The formed precipitates were centrifuged (4000 rpm, 10 min) and the residue was washed several times with distilled water and absolute ethanol.The products were dried in air at 60°C for 24 h under control environment and can be stored for extended period of time.The crystallite sizes are calculated from Scherrer equation D=Kλ/(βcosθ) [13] to be (3.56 and 1.14) nm for Mn 2+ and Ce 3+ Doped ZnS QDs respectively, i.e. the radii are (1.78 and 0.57) nm, which is smaller than Bohr radius a B , where Bohr radius for ZnS is 2.5nm, so our samples have radii smaller than Bohr radius.That means, we have quantum dot (QD) nanocrystals.So we have strong confinement in the wave function of the electron.Also, we can see that the size of Ce 3+ Doped ZnS QDs is smaller than that for Mn 2+ and Doped ZnS QDs, and this is very clear from the broadening of the diffraction peaks in (Fig. 1b), also FWHM data in Table-1 for Ce 3+ Doped ZnS are larger than that for Mn 2+ Doped ZnS.So that the surface area to volume ratio is higher with Ce 3+ Doped ZnS QDs.

EDS Analysis for Mn 2+ and Ce 3+ Doped ZnS QDs
The chemical composition of the samples is analysed by EDS.Figures 2 & 3 proved the presence Zn, S with small traces of Mn 2+ and Ce 3+ ions as depicted in the spectra.The concentration of dopants is normally too small to detect with EDS, but in this case a very small Mn and Ce peaks are observed suggesting that the amount of Mn and Ce present in the synthesized compound is high (> 1 wt %).It is obvious that the prepared nanoparticles were found to be in cluster form.The surface morphology of doped ZnS QDs have spherical shape.In some places, various sizes of the particles (small and large size) are observed, i.e. nano-sized particles seem to be randomly distributed in the films and this observation also has been seen by Kanazawa and Kamitani [14].

(I-t) characteristic for Mn 2+ and Ce 3+ Doped ZnS QDs photoconductor device
When the light was turned on, conductivity increased and after the light was turned off, the current returned to its original value.This process was repeated many times as seen from Figures (6&7).From our data, it appears to be possible to control the response of the current in a semiconducting photodetector because the electrons in the nanoparticles receive their excitation energy from the power of the light source, it is possible to "switch" these nanoparticles reversibly between higher and lower states of conductivity.As we see from the current an average of ~ms rise time τ r to travel from its lowest point to its highest and nearly ~ms fall time τ f to retreat once the light was turned off.This response rate is fast, especially for an electronic device.The gain G which is I ph /I d and Sensitivity equals to [Gx100%] are listed in Table -2

Figure of merit
Spectral photoresponses from doped ZnS devices were measured at fixed 0.5 V bias.All the measurements were performed at room-temperature and ambient environment.The photoresponsivity, quantum efficiency, Noise-Equivalent Power (NEP), Detectivity D and Specific Detectivity D * of photodetector devices can be estimated as follows:

(I) Responsivity R(λ) for ZnS PC Detector
The responsivity of the ZnS nanoparticles-based UV photodetector was obtained using the relation [15]: where I ph is photocurrent and P inc is incident UV light power, which is 61mW/cm 2 .The responsivity of the fabricated UV photoconductive detector; measured at 0.5 V applied bias voltage and 300 nm illumination with 61 mW/cm 2 light intensity; was 1.49 & 5.59 A/W for Mn doped ZnS & Ce doped ZnS UV photodetector respectively.The obtained responsivity of our device is more than those reported by Fang et al. [16] for ZnS nanobelt-based UV photodetectors which is 1x10 -26 A/W at 10 V bias voltage upon 320 nm light illumination.The spectral responsivity of the fabricated UV photodetector is shown in Fig. 8.As shown in this figure, the photoresponsivity was high in the UV region (200 -300 nm).High UV-to-visible rejection ratio can be defined as the responsivity measured at 300 nm divided by the responsivity at 450 nm [17].The obtained QE of our devices is more than those reported by Liang et al. [18] for ZnS nanowire (R λ of approximately 1.86 A W −1 and QE of approximately 7.1 × 10 2 %).The QE of the fabricated UV photodetector is shown in Fig. 9. NEP of the ZnS nanoparticles-based UV photodetector was obtained using the relation [15]: …(1) where I n is noise current (If noise from the dark current is the dominant contribution) so noise current is given by: …(2) Where: R d is resistance of detector in the dark and is bandwidth which is given by [15]: …(3) Where: equals to .The detecting capability of the detector improves as the NEP decreases.NEP of the fabricated UV photoconductive detector; measured at 0.5 V applied bias voltage and 300 nm illumination with 61 mW/cm 2 light intensity; was 1.93x10 -12 W & 0.51x10 -12 W for Mn doped ZnS & Ce doped ZnS UV photoconducive detector respectively.The NEP of the fabricated UV photodetector is shown in Fig. 10.The detectivity (D) and Specific Detectivity D * of the UV photodetector device is given by using the relations [15]: [ Under illumination at 300 nm UV light, the calculated detectivities of Mn doped ZnS & Ce doped ZnS devices (with fixed 0.5 V bias) are 0.51 × 10 12 W -1 and 1.93× 10 12 W -1 respectively as shown in figure 11.The specific detectivities of ZnS:1wt.%Mn& ZnS:1wt.%Cedevices are 0.73 × 10 12 and 2.76× 10 12 Jones respectively.Specific detectivities of our devices are much higher compared to the detectivity of previously reported photodetectors based on inorganic ZnS hybrid system (10 10 Jones) [16].

CONCLUSIONS
ZnS doped with manganese and cerium QDs with a cubic structure have been prepared successfully by a novel method using microwave irradiation.It is a simple and efficient method to produce ZnS nanoparticles with regular shape, small size and high purity.It has been fabricated a photoconductive detector successfully.The device can operate at low bias voltage under UV illumination and it can show highest sensitivity and very fast response making them having potential applications as electrical gating for binary switching.The photoresponse in ZnS:(Mn & Ce) is distinctively different from conventional semiconductor photon detectors whose photoresponse strongly depends on light wavelength and an efficiency of ~10 3 % is achieved with a biased photodetector at 0.5 V.The fabrication method studied here opens a viable route to doped ZnS optoelectronics for fast and highly-efficient photoconductive detectors.

Figure( 3 )
Figure(3)(a) SEM image for ZnS:1wt.%Ce,(b) EDS spectrum for ZnS:1wt.%Ceb) Morpgology of Mn 2+ and Ce 3+ Doped ZnS QDs.FESEM Analysis for Mn 2+ and Ce 3+ Doped ZnS QDs Figures 4 & 5 illustrate FESEM images of Mn 2+and Ce 3+ Doped ZnS films, respectively, at low and high magnifications.It is obvious that the prepared nanoparticles were found to be in cluster form.The surface morphology of doped ZnS QDs have spherical shape.In some places, various sizes of the particles (small and large size) are observed, i.e. nano-sized particles seem to be randomly distributed in the films and this observation also has been seen by Kanazawa and Kamitani[14].

Figure( 6 )Figure( 7 )
Figure(6) Photoresponse time of the fabricated ZnS:1wt.%MnQDs UV photodetector upon exposure to 360nm light at 0.5V bias voltage, the repeatability property(ON/OFF) with the sensitivity of UV detector for 8 s.