Symmetry-Defying Iron Pyrite (FeS2) Nanocrystals through Oriented Attachment

Iron pyrite (fool's gold, FeS2) is a promising earth abundant and environmentally benign semiconductor material that shows promise as a strong and broad absorber for photovoltaics and high energy density cathode material for batteries. However, controlling FeS2 nanocrystal formation (composition, size, shape, stoichiometry, etc.) and defect mitigation still remains a challenge. These problems represent significant limitations in the ability to control electrical, optical and electrochemical properties to exploit pyrite's full potential for sustainable energy applications. Here, we report a symmetry-defying oriented attachment FeS2 nanocrystal growth by examining the nanostructure evolution and recrystallization to uncover how the shape, size and defects of FeS2 nanocrystals changes during growth. It is demonstrated that a well-controlled reaction temperature and annealing time results in polycrystal-to-monocrystal formation and defect annihilation, which correlates with the performance of photoresponse devices. This knowledge opens up a new tactic to address pyrite's known defect problems.

and crystallinity of the FeS 2 nanocrystals. The OA growth is observed in creation of four different shapes of FeS 2 nanocrystals (cube, sheet, hexagonal plate and sphere) implying this is a dominant mechanism for FeS 2 nanostructures. Observing an OA growth mechanism could offer insight into pyrite's known problems that have been attributed to vacancies and crystal defects that hold it back as a highly promising photovoltaic material 28 . High-resolution transmission electron microscopy (HRTEM) images show the progression from initial seeds to final monocrystal phase. To our knowledge, the OA growth has not been reported utilizing a hot-injection method, as usually a precipitation method is used to create the initial seeds and the final crystals. Finally, it is shown that FeS 2 sheets created from the OA growth process can be integrated into a photodetector device and can be used as a probe for defect mitigation, and more importantly, shows the extent of recrystalization's effect on optoelectronic performance.
In the following report, evidence will first be presented for the OA growth in multi-shaped FeS 2 nanocrystals and a proposed reasoning for final shape created in the nanocrystals. Characterization of FeS 2 nanocrystals will be presented next followed by tuning of the nanocrystal size by utilizing OA kinetics. Finally, the performance of photodetector devices created out of FeS 2 pyrite nanosheets will be presented and analyzed.

Results
The initial step in synthesis of FeS 2 nanocrystals consists of the creation of FeS 2 quantum dot (QD) seeds. QD formations are realized by a rapid hot-injection of sulfur into an iron precursor solution, quickly creating QDs which show an average diameter of 2 nm with a narrow size distribution (Figure 1 and Supplementary  Fig. S1) and create a transparent deep blue solution when dissolved in chloroform. Figure 1b shows the optical absorption spectrum of FeS 2 QDs which exhibit strong quantum confinement and well-defined excitonic features, that match well with a previous report 29 . The OA process then proceeds utilizing the QDs as primary particle seeds. By controlling the injection temperature, different surface facet-rich nanocrystals can be obtained, which directs the collision or the attachment direction and thus control the cube or symmetry-defying sheet growth.
Since different surface facets of FeS 2 QD seeds exhibit different surface energy, anisotropic OA growth is realized by the combination of energetically unfavorable surface facets which will reduce the overall energy of the formed FeS 2 nanocrystals. After the aggregation occurs (See Supplementary Fig. S2), the OA process continues with the formation of a polycrystalline structure followed by a recrystalization to a monocrystal. TEM images of each step for FeS 2 nanocube formation are presented in Fig. 2a-2d. Optical absorption spectrum tracking structure changes are presented in Supplementary Fig. S3. Note that by the aggregation step, a cube-like shape can already be seen being formed (Fig. 2b). The OA growth is defined by the material's symmetry and the surface facets of FeS 2 which exhibit the lowest energy 30 . By increasing the injection temperature from 393 K to 418 K, thin FeS 2 pyrite {100} nanosheets are formed for the first time by the OA mechanism ( Fig. 2e-2h). The small seeds can be seen within the sheet-like matrix (Fig. 2f), reminiscent of PbS sheets formed by OA growth 31 . In the case of the nanosheets, it is seen that   final sheets grow thicker from aging (Fig. 2h). Supplementary Figure  S4 presents the thickening evidence through the TEM cross section of the sheets at different growth time.
We interpret the symmetry-defying OA growth mechanism of our FeS 2 nanocrystals based on the thermodynamic stability of different surface facets predicted by Barnard and Russo 30 . In their work, it is shown a truncated FeS 2 nanocluster of 5 nm is made up with 6 {100}, 8 {111} and 12 {110} surface facets. Figure 3 presents a depiction of these nanoclusters and the paths to different shape formation seen in this study. In the case of cube growth (path A), a relatively larger FeS 2 QD seed is created at a lower injection temperature, which results in mainly {100} surface planes being formed. The FeS 2 QD seeds are stabilized by the OA preferentially along {100} facets to form cubic FeS 2 nanocrystals with {100} surface planes. Regarding the FeS 2 nanosheet formation, creation of relatively smaller crystallites with higher {110} surface area explains the in-plane attachment. FeS 2 QD seeds with {110}-rich surfaces are created when the temperature of the injection is increased. We interpret the thin FeS 2 nanosheet formation by the aggregation of the seeds through the {110} surface plane, shown in Fig. 3, pathway B. Since {110} surface facets of FeS 2 seeds have higher surface energy, they are preferentially consumed by the in-plane 2D attachment, resulting in the FeS 2 nanosheet formation. Conversion to the thicker sheet structures most likely occurs through attachment of the sheets prevalent {100} surface, as the planar dimension does not change in size, shown in Supplementary Fig. S4. The ability to control initial seeds and their surface facets will prove valuable in extracting the obtainable shapes of FeS 2 nanocrystals (see Supplementary Fig. S5), which have shown vastly different properties in optoelectronic and electrochemical devices by us (this will be discussed in a future report).
It has been observed and explained kinetically that in the OA growth model, higher growth temperature leads to smaller particles due to the extra energy allowing for easier de-adsorption of the particles during the collision step of the OA based growth 32 . The OA controlled tunability of FeS 2 nano-crystal dimensions is confirmed by varying growth temperature of the cubic synthetic route. Figure 4a-4c shows TEM images of FeS 2 nanocrystals when the growth temperature was 493, 523 and 543 K, respectively. Quantitatively, it is seen that as the growth temperature increases, the average size of the final FeS 2 nanocrystal decreases from 64 nm, 43 nm, and 23 nm, respectively, providing additional evidence of an OA controlled growth mechanism. Another key difference between OR and OA growth in nanocrystals is particle size dependence on the growth time. In OR, as stated above, bigger particles grow at the expense of smaller particles, making size increase as time progresses. In OA, the particles attach to create a more stable particle, and then usually cease to grow afterwards (there exists cases where after an OA step, OR takes over and some growth still occurs). This leads to a stagnation of size after the OA growth has taken place. In this study, there exists a point where the FeS 2 nanostructures stop growing in size. Supplementary Figure S6 shows the size of cubic structures at 40 min and 120 min into the synthesis and there is no observed change in the overall sizes. Controlling the size of FeS 2 nanoparticles is an important goal, as it has been stated that only a 40 nm film is required in devices due to the material outstanding absorption coefficient 12 .
The existence of different size and shape of FeS 2 nanocrystals suggests different collision and coalescence behavior of FeS 2 seed crystallites. In the OA growth process, the reaction temperature dominates the collision and the coalescence which is attributed to the particle's medium-and short-range interactions, such as Van der Waals forces and dipole-dipole interaction forces. Van der Waals forces are estimated to be less than 0.5 RT, which is not enough to stabilize superstructures under ambient conditions 33 . The force capable of producing FeS 2 polycrystals is thus believed to be the long range dipole-dipole attraction. The energy of dipole attraction between FeS 2 QD seeds can be calculated using the classical formula ,V. 5 2m 2 /2pe o r(r 2 2 d NP 2 ). Estimating the center-to-center interdipolar separation r to be 2.6 nm, the FeS 2 QD seed diameter to be d NP 5 2 nm and taking the dipole moment for this size m 5 17.6D, the energy of dipole attraction ,V. is equal to 5.2 kJ/mole 34 . In the weakly flocculated colloidal state, the dipole-dipole potential can also be expressed as a function of temperature T. When dipoledipole potentials ,V. and kinetic energy (KE) are plotted as a   function of T (Fig. 4 (d)), an intersection represents a critical temperature, T c of the system at 416 K. T c represents when the thermal energy exceeds the attractive potential energy among FeS 2 seeds. If the reaction temperature is lower than the T c , the attractive dipoledipole potential energy dominates the OA process by coalescence. Once the reaction temperature exceeds T c , the KE will control the OA growth, which is dictated by the collision. The size of FeS 2 nanocrystals will be controlled by reaction time in the coalescence state. In our FeS 2 synthesis, we control the coalescence and collision to yield FeS 2 nanocubes with different sizes, by tuning the heating rate and thus the reaction time within the coalescence state (Fig. 4a-4c).
While the shape and size control is an important goal in the FeS 2 system, defect mitigation may be the most crucial aspect in achieving optimal FeS 2 nanostructures. It has been widely accepted that the defects (such as, surface states, dislocations, twins, etc) of FeS 2 nanocrystals dictate their optoelectronic and electrochemical applications, therefore a strategy to achieve high quality crystalline FeS 2 needs to be identified. Polycrystalline-to-monocrystalline conversion of the OA growth can be utilized to create highly crystalline FeS 2 nanocrystals. Figure 5a shows a HRTEM image of a FeS 2 nanocube at 40 min into the synthesis. It can be seen that different domains (outlined by lines) exist, while stacking faults can clearly be seen (highlighted by arrows) due to the collision of the OA growth process. These defects are detrimental to material quality as they act as the charge recombination centers for excitons and need to be eliminated to create optimal solar cell devices. Upon greater lengths of aging time in the same pyrite solution, it is seen that these defects are eventually eliminated. Figure 5b shows a monocrystalline cubic FeS 2 nanocrystal aged for 120 min and the inset shows [100] growth diffraction pattern. This suggests that longer aging times will be beneficial for FeS 2 nanomaterial, due to the stagnation of the OA controlled FeS 2 growth, the longer aging times should not interfere with shape/size.

Discussion
As stated above, it is widely agreed that defects in pyrite material is the limiting factor for performance of devices 2,[35][36][37] . In this study, it is seen that the pyrite particles eventually reach a maximum size, and then begin to convert from poly-crystalline to mono-crystalline, which will reduce the defects that are caused by the OA mechanism. To test this hypothesis, a series of photodetector devices were fabricated, using FeS 2 nanocrystals with varying aging times to examine the effect of crystallinity on the device performance. The timedependent photoresponse of FeS 2 nanosheets are shown in Fig. 6 (a). The current difference between irradiation (light on) and dark (light off) is clearly enhanced by increasing the aging time of the FeS 2 nanosheets. The figure of merit we use to compare photodetector performance is the normalized detectivity (D*) 38 . D* values of the FeS 2 nanosheet devices are 5.84 3 10 10 , 8.60 3 10 10 , and 1.85 3 10 11 Jones, corresponding to 10 min, 40 min, and 240 min aging time of nanosheets, respectively (shown in Fig. 6(b)). Since FeS 2 nanosheets demonstrate a strong absorbance in the near infrared (NIR) wavelength, they could work as the NIR photodetector. Figure 6b shows the performance under 1000 nm illumination, which confirms excellent NIR performance and again demonstrates the effect of crystallinity on the photodetector performance. The R l and D* of   increased aging time during the FeS 2 growth. These results support that defects within the material are being mitigated due to the recrystallization of the FeS 2 nanomaterials. More detailed photodetector device studies are underway to utilize their unique IR absorbance.
A symmetry-defying OA growth and its implications for different shaped FeS 2 nanocrystals have been presented and discussed. FeS 2 nanocrystals show the growth starting with FeS 2 QD seeds, which exhibit excitonic absorption behavior and enable further OA growth for shape and size control. A growth pathway model and thermodynamic reasoning are then presented to facilitate understanding of shape and size control in the FeS 2 system. Shape and crystallinity of FeS 2 nanocrystals is shown to be dependent on reaction temperature and aging time. Photodetector performance is shown to be correlated with crystallinity, offering support for defect mitigation in the material. Observation of the symmetry-defying OA growth in FeS 2 nanocrystals and its effect on crystallinity will facilitate FeS 2 along on its path to becoming a ''golden'' material for sustainable energy applications. Controlling crystallinity is a key point in the generation of complex functional nanomaterials. Self-assembly of particles into larger single-crystalline objects by the OA mechanism, is one of the most promising approaches in nanotechnology. This OA evolution process can be adjusted by cosolvents 26,31 , high pH value 34 , temperature and time 24 . A well-controlled reaction conditions in the OA process can facilitate the high quality nanocrystal growth.

Method
Synthesis. The FeS 2 nanocube synthesis starts with 0.5 mmol FeCl 2 in octadecylamine (ODA, 12 g) loaded into a three neck flask and degassed and back filled with argon, heated to 393 K and allowed to decompose for 120 min. Another three neck flask is then loaded with 4 mmol sulfur powder in diphenyl ether (5 mL), is degassed and back filled with argon, and heated to 343 K for 1 hour to dissolve. The sulfur solution is then quickly injected into the Fe-ODA precursor at the temperature of 393 K. After injection, the combined solution was heated to 493 K and aliquots at different time intervals were taken for UV-vis-NIR absorption test and HRTEM characterization. For FeS 2 thin sheets, injection temperature of the Fe-ODA precursor is raised to 418 K with everything else kept the same. Particles were separated by centrifugation and purified by being re-dissolved and crashed in chloroform-methanol. The final particles were dispersed in chloroform for storage and characterization.
Materials Characterization and Devices fabrication. All UV-Vis-NIR absorbance spectra were obtained on a UV-3600 Shimadzu Spectrophotometer. HRTEM images were obtained using Field Emission FEI Tecnai F20 XT. The photodetector devices are fabricated as following: A PEDOT:PSS layer is used to flatten the ITO patterned glass substrate and serves as a hole transporting layer. The FeS 2 nanosheets were dissolved in chloroform with a concentration of 25 mg/mL. The FeS 2 nanosheets were deposited on the PEDOT:PSS surface by spin coating method at the speed of 1500 RPM, Then, a thin layer of calcium (,10 nm) was thermally evaporated. Finally, a patterned aluminum electrode (,80 nm) was evaporated on the top surface of the calcium, completing the device.