Phase-pure iron pyrite nanocrystals for low-cost photodetectors
© Liu et al.; licensee Springer. 2014
Received: 29 July 2014
Accepted: 25 September 2014
Published: 2 October 2014
Earth-abundant iron pyrite (FeS2) shows great potential as a light absorber for solar cells and photodetectors due to their high absorption coefficient (>105 cm-1). In this paper, high-quality phase-pure and single crystalline pyrite nanocrystals were synthesized via facile, low-cost, and environment friendly hydrothermal method. The molar ratio of sulphur to iron and the reaction time play a crucial role in determining the quality and morphology of FeS2 nanocrystals. X-ray diffraction and high-resolution transmission electron microscopy confirm that phase-pure and single crystalline pyrite nanocrystals can be synthesized with high sulphur to iron molar ratio and sufficient reaction time. For the first time, a crystalline nanogap pyrite photodetector with promising photocurrent and UV-visible photoresponse has been fabricated. This work further demonstrates a facile route to synthesize high-quality FeS2 nanomaterials and their potential in optoelectronic applications.
Iron pyrite (cubic β-FeS2), commonly known as a non-toxic and earth-abundant compound, has been regarded as one of the most promising semiconductor materials to meet the urgent demand for cost-effective energy solutions [1, 2]. FeS2 has a band gap of 0.95 eV, which matches the solar spectrum, high absorption coefficient (approximately 105 cm-1 for hν > 1.3 eV) [3, 4], excellent electric properties with carrier mobility about 360 cm2 V-1 s-1, and long minority carrier diffusion length (approximately 0.1 to 1.0 μm) [1, 2]. It provides a new alternative way for high-performance photovoltaic cells as well as optoelectronic devices.
Despite these attractive properties, the promises of FeS2 have not been fulfilled. For example, the conversion efficiency of FeS2 solar cells has been limited to only 3% and further improvement remains challenging . The main issues to synthesize high-performance FeS2 devices are phase impurities and surface defects, which could greatly undermine the superior properties of FeS2. Although high quantum efficiency (>90%) and photocurrent (>42 mA cm-2) have been reported for FeS2 solar cells [7–10], the poor crystal quality of bulk FeS2 has led to very low open circuit voltages (<0.2 V) .
The recently realized high-quality FeS2 nanostructures have triggered the new interest for their applications in various types of devices, such as solar cells, photoelectrochemical cells, photodetectors, and battery cathodes [11–14]. To date, various ways to synthesize FeS2 nanostructures have been reported, including metal-organic chemical vapor deposition , thermal sulphidation , magnetron sputtering , hydrothermal synthesis , and hot injection method . Among these methods, hydrothermal method has been favored due to its low temperature process, which can greatly reduce the phase impurities and surface defects .
In this work, we demonstrate a polymer-assisted hydrothermal method without using any expensive precursors or poisonous reagents to synthesize nanostructured FeS2, including FeS2 polygonal nanoparticles, nanocubes, and hierarchical nanostructures. In addition, a nanogap (with a gap as small as 200 nm) FeS2 photodetector has been fabricated. Using such a simple nanogap photoconductor, promising photocurrents and UV-visible (UV-vis) spectral photoresponse have been observed. This facile method to synthesize high-quality FeS2 nanomaterials and their potential applications in high-performance optoelectronics devices demonstrates the growing potential of this earth-abundant material towards low-cost optoelectronic applications.
To obtain high-quality FeS2, the synthesis was carried out using different reaction recipes. All reagents used in our work are of analytical grade from J & K Scientific (Edwardsville, Nova Scotia Canada). Firstly, gelatin of 0.54 g was dissolved in 30 mL hot deionized (DI) water. The gelatin here can be easily adsorbed onto Fe(OH)2, thus providing an encapsulation for FeS2 nanocrystal during the reaction. In this way, it can prevent the diffusion of S2- ions, S and H2S to the surface of Fe(OH)2, and the aggregation of nanoparticles into large microparticles . Therefore, gelatin plays a key role in the size uniformity and stabilization of FeS2 nanocrystals. Secondly, 1.5 mmol FeCl2 · 4H2O was dissolved in 5 mL DI water and then added to the gelatin solution drop by drop at room temperature to avoid the oxidation of Fe2+. By adding NaOH powder, the pH of the solution was then slowly adjusted to about 12. The overdosing OH- at this stage provides an alkaline environment, thus facilitating the reaction processes. NaOH has a significant influence on the reaction between S and water and hence the quality of FeS2 nanocrystals . During this process, the transparent solution changed from light yellow to light green gradually, and then separated out into dark green flocculent precipitates. Lastly, sulphur powder was added to the homogenous solution, which was magnetically stirred for over an hour. The final concentration of gelatin was about 1.5% w/v. The prepared mixture was sealed in a stainless steel autoclave and maintained at 200°C for a certain reaction time before being naturally cooled down to room temperature. The black product was then centrifuged and washed using DI water and alcohol for several times to remove the excess polymer and ions . The phase-pure and crystalline nanocrystals was then acquired and dispersed in ethanol to avoid oxidation.
The experimental parameters used for the hydrothermal synthesis of FeS 2 nanoparticles
Reaction time (hour)
Phase and impurities
FeS2 + Fe x O1 - x + S
FeS2 + Fe x O1 - x + S
FeS2 + Fe x O1 - x
FeS2 + Fe x O1 - x
Nanocubes and polygonal nanoparticles
FeS2 + Fe x O1 - x
Nanocubes and hierarchical particles
Results and discussion
In conclusion, single phase pyrite FeS2 nanocrystals were successfully synthesized using a facile hydrothermal approach. The high-quality crystalline pyrite FeS2 nanocrystals were further confirmed by HRTEM and XRD measurements. The sulphur and iron molar ratio, [S]/[Fe], plays a critical role in nanocrystal quality and morphology. A nanogap pyrite FeS2 nanocrystal photodetector was fabricated using standard photolithography and focused ion beam milling. The nanogap photodetector shows a very high photocurrent in the range of 10-2 to 1 μA for approximately 1 μm2 gap area and spectral response in the UV-vis range. The facile approach for pyrite FeS2 synthesis and the successful demonstration of nanocrystal photodetector suggest a promising way to achieve low-cost optoelectronic devices using pyrite FeS2 nanocrystals.
This work was supported in part by the National Basic Research Program (973) of China through Grant 2013CB933301 and the National Natural Science Foundation of China through Grants NSFC-51272038 and NSFC-61204060.
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