Why does CuFeS2 Resemble Gold? - The Journal of Physical

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Why does CuFeS2 Resemble Gold? Anumol Sugathan, Biswajit Bhattacharyya, V. V. Ravi Kishore, Abhinav Kumar, Guru Pratheep Rajasekar, D. D. Sarma, and Anshu Pandey J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b03190 • Publication Date (Web): 17 Jan 2018 Downloaded from http://pubs.acs.org on January 18, 2018

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The Journal of Physical Chemistry Letters

Why does CuFeS2 Resemble Gold? Anumol Sugathan╪1, Biswajit Bhattacharyya╪ 1, V. V. R. Kishore1, Abhinav Kumar1‡, Guru Pratheep Rajasekar1, D. D. Sarma1 and Anshu Pandey1* 1

Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012

╪

These authors contributed equally to this work

ACS Paragon Plus Environment

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ABSTRACT

While several potential applications of CuFeS2 quantum dots have already been reported, doubts regarding their optical and physical properties persist. In particular, it is unclear if the quantum dot material is metallic, a degenerately doped semiconductor or else an intrinsic semiconductor material. Here we examine the physical properties of CuFeS2 quantum dots in order to address this issue. Specifically, we study the bump that is observed in the optical spectra of these quantum dots at ~500 nm. Using a combination of structural and optical characterization methods, ultrafast spectroscopy, as well as electronic structure calculations, we ascertain that the unusual purple color of CuFeS2 quantum dots as well the golden luster of CuFeS2 films arise from the existence of a plasmon resonance in these materials. While the presence of free carriers causes this material to resemble gold, surface treatments are also described to suppress the plasmon resonance altogether.

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CuFeS2 quantum dots(QDs) have attracted considerable interest over the past years owing to their infrared band gap as well as their benign, economically sustainable chemical composition.12

In particular, these materials have been posited as earth-abundant, environmentally friendly

replacements for lead, cadmium and other heavy metal based QDs.3-4 Significantly, bulk CuFeS2 is a naturally occurring mineral that is recognizable by its characteristic golden luster.5-6 It is known to crystallize in a tetragonal lattice.7 In its bulk form CuFeS2 is a semiconductor with a band gap of 0.52 eV.8-10 CuFeS2 QDs have been shown to exhibit a strongly size dependent optical band gap that is tunable from 0.52 – 2 eV,9 that is well suited for opto-electronic applications in the near-infrared. CuFeS2 is also unique among the lighter

semiconductors

in

exhibiting

properties

such

as

thermoelectricity11-14

and

ferroelectricity,15 making it suitable for a host of energy generation, conversion and harvesting applications.16 Despite its promise, the realization of the full potential of CuFeS2 QDs is stymied by uncertainty regarding its physical properties. In particular, although CuFeS2 is a semiconductor, its bulk and nanoscale forms bear an uncanny resemblance to metallic gold. For example, as shown in Figure 1a, a solution of CuFeS2 QDs has a distinctive purple coloration, similar to gold nanocrystals. Further, as shown in the right panel, freshly cast films of CuFeS2 QDs on glass have a metallic golden luster. Optically, this coloration arises because of the existence of a prominent band at ~500 nm as shown in Figure 1b. This optical feature, henceforth termed A2, is located at 2.45 eV, significantly higher than the optical band gap of the material at 0.56 eV. We further define the energy A1 as being 0.1 eV higher than the optical band gap, i.e. at 0.66 eV for this particular sample. We observed that the position of the A2 feature remains almost constant during the nanocrystal growth in sharp contrast with the band gap, which shifts considerably. This is


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illustrated in Figure 1c. Figure 1d demonstrates the variation in the A2 position with band gap of the sample. It is evident from the figure that the position of the A2 feature does not undergo any considerable change with the band gap and hence size of the sample. The inset shows the evolution of the A2 feature with growth time of the sample. In this article, we study the optical, electrical and structural properties of CuFeS2 QDs with a view to determine the precise nature of this particular feature. Past studies have variously attributed the spectroscopic properties of this material to plasmons or to d-band absorption.6,

17-18

A material with sufficient free carriers to

give rise to a plasmon resonance may find uses in generating local field enhancements or else as a conductive contact.19-22 In contrast, the existence of distinct bands motivates the investigation of a material for light harvesting, particularly towards exceeding the Shockley-Queisser limit.2325

In particular, a material with intermediate bands offers the possibility of allowing electron

extraction at two different voltages and thereby acting as a multijunction in suitable device geometries. Given the vastly different implications of either interpretation, a complete understanding of the origin of this band is necessary to fully realize the potential applications of this material. We employ a combination of static and transient optical spectroscopy to study the nature of the 500 nm optical band. Our optical studies are correlated with the structural properties of this material as analyzed through X-ray diffraction (XRD), X-ray photoemission (XPS) as well as composition. The inferences drawn from this analysis are further supported by computational studies to simulate the band structure of CuFeS2. Finally, an independent verification of our conclusions is obtained through electrical and electro-optical characterization of CuFeS2 QD films.


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Figure 1. (a) CuFeS2 QDs dispersed in hexane show a characteristic purple color (left) and CuFeS2 QD films (right) exhibit a golden luster. The reflection of an overhead fluorescent is visible on the lower part of the film. (b) Optical absorption spectrum of CuFeS2 QDs with distinctive feature at 500 nm, labeled as A2 and 0.1 eV above band edge labeled as A1. (c) Variation in optical band gap of CuFeS2 QDs and the A2 feature energy with growth time. (d) Variation of A2 position with optical band gap. Inset: Exemplary absorption spectra of CuFeS2 QDs during different stages of growth (7 minutes, green, 10 minutes, red and 15 minutes, black).


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Consistent with previous reports,6,

9, 17-18

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the as-synthesized dots are seen to crystallize in a

tetragonal chalcopyrite structure, similar to bulk CuFeS2. This is exemplified in the X Ray Diffraction (XRD) patterns shown in Figure S1a. The bulk powder patterns are shown by the green line at the bottom of this figure panel. Impurity phases are not evidenced in the XRD patterns during any stage of the growth. Transmission electron micrographs are consistent with a homogenous ensemble (Fig. S1b and S1c). Furthermore, the XRD result was confirmed at a single particle level using High Resolution Transmission Electron Microscopy (HRTEM) imaging (Fig S1d-g). We therefore infer the existence of a homogenous ensemble of CuFeS2 QDs with a tetragonal chalcopyrite structure. The ratio of the optical absorbances of the A2 and A1 features is roughly 40 in the example shown in Figure 1b. If A2 and A1 were to originate from two different materials with similar cross sections, the relative abundance of the material or phase giving rise to the A2 feature would be about an order of magnitude greater than the abundance of the material giving rise to A1. As we do not observe any evidence of any nonCuFeS2 phase in XRD as well as in HRTEM, the origin of the A2 feature in CuFeS2 QDs is thus interpreted to be an intrinsic property of CuFeS2 QDs, and not an outcome of a sample inhomogeneity or extraneous crystalline phases. We firstly note that the presence of an enhanced optical absorption in the vicinity of the A2 feature is expected from first principles computations. For these studies, the electronic structure of CuFeS2 is calculated with a plane-wave PAW implementation26-27 of density functional theory using Vienna Ab-Initio Simulation Package.28-30 The experimentally reported tetragonal structure (space group I4̅2d (122)), with a = b = 5.289 Å and c = 10.423 Å has been considered and the internal positions have been optimized until the forces on each atom were less than 0.001 eV/Å. The general gradient approximation (GGA)31-32 has been used for the exchange correlation


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functional and a Hubbard correlation term 33-34 Ueff = U − J = 2.2 eV, has been considered for the Fe (3d) states. A plane wave cut-off energy of 500 eV has been used for the calculations, with a k-point grid of 4x4x2. We used VASPKIT for postprocessing of the VASP calculated data and obtained the optical absorption results. From our results we have found that the antiferromagnetic (AFM) configuration with the spins on any two Fe atoms bonded to a common S atom are opposed and directed along the c axis (Figure 2a) is the ground state. The origin of this magnetic structure can be understood in terms of Fe-S-Fe super exchange coupling for Fe ions in successive planes along the c-axis in analogy to the closely related antiferromagnetic ground state of NiS in NiAs structure type.35

Figure 2: (a) CuFeS2 structure with Antiferromagnetic configuration spins on Fe atoms. (b) Density of states for CuFeS2 using GGA+U with Hubbard U correction 2.2 . (c) Optical absorption for CuFeS2 using GGA+U with Hubbard U correction 2.2 . The magnetic moment on Fe is found to be 3.5 µB while Cu and S have zero magnetic moments, which indicates that the oxidation states of Cu, Fe and S are +1, +3 and -2 respectively. In our problem CuFeS2 considered as a 4 formula unit structure which has been given rise to 3.5 µB magnetization, so calculated magnetization per formula unit will be 0.875 µB. The significant


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reduction in Fe magnetic moment (expected to be ~ 5µB) is supposed to be a consequence of high degree of covalency between Fe and S.10 We then calculated the density of states (Figure 2b) which shows that the bandgap obtained from theoretical calculations is 0.53 which is very close to the experimentally obtained value (0.52 eV). Using this methodology, we further calculated the optical absorption of CuFeS2 (Figure 2c). As shown in this figure, the simulated optical spectrum of CuFeS2 which shows a peak at approximately 2 eV, in the vicinity of the A2 feature (2.45 eV). This absorption is inferred to arise from the cationic d levels. It is however noteworthy that whereas the empirically observed ratio of optical absorbances at A2 and A1 are 40, theoretical calculations yield a smaller value of 18 (A1 and A2 have taken as 0.66 eV and 2.45 eV). We further note that the A2:A1 ratio does not change significantly with the growth time of the material. For example, as the growth time increases from 7 to 30 minutes, the ratio changes only slightly from 38 to 48. This discrepancy suggests that the optical absorbance of CuFeS2 may not be fully explained by a simple band to band absorption picture, and an additional investigation of the material is necessary. In order to diagnose the origin of the A2 feature, we firstly studied the dielectric dependence of the optical absorption of CuFeS2 QDs. It is well known that localized surface plasmon resonances (LSPRs) of nanocrystals have a strong dependence on their dielectric environments. In the past, this dependence has been used to sensitively detect analytes as well as sense the homogeneity of dielectric environments around metal nanocrystals.36-37 In our study, this dependence offers a sensitive route towards unraveling the intrinsic nature of CuFeS2 QDs. We firstly observed subtle solvent dependent changes in the CuFeS2 QD optical spectra. These are highlighted in Figure 3a. It is apparent that a change in the refractive index of the medium dielectric from 1.37 (hexane) to 1.51 (tetrachloroethylene) leads to a shift of 0.024 eV. However,


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as these CuFeS2 QDs still retain a shell of ligands, it is evident that these results are only of qualitative significance. In order to improve the reliability of these data, we studied the response of CuFeS2 QD films. Films of CuFeS2 QDs were deposited onto glass. It was observed that thick CuFeS2 QD films show a strong shifting and broadening of the A2 feature. A further increase in the extinction in the 1000 nm region is noted (see Figure 3b). It is noteworthy that these changes do not occur along with a commensurate shift in the optical band gap of the QDs at 0.63 eV. While this differential response of the A2 feature as well as the band edge of the CuFeS2 QD optical spectrum is consistent with different origins to the two features, this is none the less a hindrance to studying the dielectric properties of these QDs. We therefore focused on preparation of sparse films of CuFeS2 QDs where inter-particle interactions are minimized. CuFeS2 nanoparticles dispersed in hexane were dip-coated onto an APTES ((3-aminopropyl)triethoxysilane) functionalized glass strip 9 mm wide. This immobilized the nanoparticles on the glass substrate. The film was then washed thoroughly with ethanol to remove the physisorbed nanoparticles. Typically, we used films with OD 0.08 at 2.48 eV for further studies. The ligands around these QDs were subsequently removed and replaced with shorter ones in order to maximize the influence of the dielectric medium. This was done by treating the films to a solution of 1:4 butylamine: propan-2-ol.38 The spectrum of such a cleaned, dried film is shown in Figure 3c (red curve). These films were now immersed in different solvents (acetone, chlorofrom, toluene and tetrachloroethylene) to study the effects of dielectric on the spectral position of the A2 feature. The position of the peak maximum is plotted against the solvent refractive index in Figure 3d. It is evident that these data are consistent with a linear dependence of the feature maximum on the


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medium dielectric, with the peak of the feature red shifting with increasing refractive index of the medium. A slope of -0.14 eV/refractive index unit (RIU) is observed.

Figure 3. (a) Absorption spectra of CuFeS2 QDs dispersed in various solvents. (b) Film and solution phase absorbance of CuFeS2 nanoparticles on a log scale. Inset: same data plotted on linear scale. (c) Absorption spectra of CuFeS2 films immersed in various solvents. (d) Peak maxima positions plotted against the refractive indices of the media around the film. In the past, similar dielectric induced shifts have been reported for metallic nanoparticles.39-41 In particular, Chen et. al. reported shifts of 44 nm/RIU for the localized surface plasmon resonances


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of 15 nm diameter gold nanoparticles (~-0.2 eV/RIU) that are comparable to the shifts observed by us in the case of the A2 feature. We further note that our measurements involve comparably smaller 5.2 nm CuFeS2 QDs, and a reduced sensitivity to the dielectric environment is expected for smaller particles. In general, for particles significantly smaller than the wavelength of light, a resonance is expected when the polarizability of the particle exhibits a pole. For a sphere, this condition reduces to 2 .41 Here ε' is the optical wavelength dependent real part of the nanoparticle dielectric constant and n is the refractive index of the medium. A change in the refractive index of the medium thus leads to a spectral shift of the resonance to a wavelength where the resonance criterion is again satisfied. While there is no specific reason to expect a linear relationship between the resonance energy and the refractive index, such a relationship is nevertheless obtained due to the limited range of medium refractive indices that can be sampled in an experiment. The observation of the strong sensitivity of the A2 feature to the medium refractive index has two key implications. Firstly, this implies that the real part of the CuFeS2 QD dielectric constant bears a negative sign. It is noteworthy that the real components of the dielectric constants of semiconductors are usually positive,37,

41

and this behavior is more typical of

plasmonic systems. Further, the negative sign of the slope observed in figure 3d is consistent with an increasingly negative value of () for lower frequencies of light. This behavior is also consistent with the properties expected from a metallic system or a degenerately doped semiconductor.37, 42-43 To further confirm this assignment, we studied the transient absorption dynamics of CuFeS2 QDs. In these studies, CuFeS2 QDs were dispersed in a hexane solution and illuminated by a 400 nm pump obtained by doubling the fundamental output of a Coherent Libra 100 fs laser. The


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transient absorption dynamics was then probed by a broadband white light probe derived from a sapphire plate. Significant bleaching of the 500 nm feature is observed (Figure 4a). As shown in Figure 4b (red dot), the bleach feature decays in a bi-exponential fashion with a time constants of 0.46 ps and 10.5 ps. The bleach observed in Figure 4b corresponds to the pumping of 0.4 excitons per CuFeS2 QD. Multiexciton formation is unexpected at this low fluence, and the two time constants must therefore correspond to the existence of two different cooling phenomena in the QDs. We further note that following electronic cooling, no photoinduced absorption is observable.44 Past studies on other materials such as CdSe have suggested a connection between poor surface passivation and photoinduced absorption in transient spectroscopic measurements.45 Our data are thus consistent with a well passivated CuFeS2 QD ensemble. We further contrast these results with the data observed in the case of CuFeS2/CdS QDs.9 In the case of CuFeS2/CdS QDs, it is clear that the A2 feature becomes indistinct following CdS shell growth (Figure 4c, red curve). This may arise either from the suppression of this feature altogether or else because of the masking of this feature from the CdS shell absorption that begins at ~510 nm. These two possibilities can be distinguished by transient absorption spectroscopy. In particular we were unable to detect optical bleach in the 2.5 eV regions over a 100 ps (Figure 4c, black dot) timescale even though a bleach is observed in the case of CuFeS2 QDs. The lack of measurable bleach is consistent with the suppression of the A2 feature in CuFeS2/CdS core shells. It is further apparent that in CuFeS2/CdS QDs, the band edge bleach builds up rapidly within a sub-picosecond time constant (Figure 4d) significantly different from the cooling dynamics observed in the case of CuFeS2 QDs (Figure 4b, red dots). For comparison, Figure 4b further shows the absence of any measurable bleach feature in the case of CuFeS2/CdS QDs on a similar timescale (black dots). We note that a property such as sub-band absorption


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cannot be suppressed because of lack of surface access. Instead, a feature such as a localized surface plasmon resonance (LSPR) may be extremely sensitive to the passivation of the QD surface, while at the same time exhibit a cooling dynamic substantially different from intraband cooling. In particular, a LSPR is expected to show three cooling rates, corresponding to dephasing (order of 10 fs, unresolved in our experiment), electron-lattice as well as electronsurface thermalization (order of 1ps) and finally, the thermalization of the lattice with the environment (order of 10 ps).21, 46 It is thus evident that the properties of CuFeS2 QDs are akin to a degenerately doped semiconductor.47 In particular, while the material does exhibit an optical band gap, it simultaneously supports a plasma oscillation. As CuFeS2 QDs do not contain intentionally introduced dopants, the role of the surface on the electronic properties is likely, and motivates a study of effects of passivation on the properties of these QDs.


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Figure 4. (a) Transient bleach feature in the time scale of 0-10 ps with a strong absorption in 2.5 eV range (blue line) of pure CuFeS2. (b) Bleach dynamics of pure CuFeS2 at 510 nm (red dot) and Cadmium coated CuFeS2 at the same wavelength (black dot). (c) Bleach feature of cadmium coated CuFeS2 (black dot) with absorbance spectra of the same sample (red line). (d) Band bleach builds up of CuFeS2/CdS at 580nm (black dot with fitted black line).

We employ a low temperature cadmium ion treatment to partially substitute the surface ions. Briefly, a solution of 0.025 mmols of CuFeS2 QDs dissolved in 3 mL toluene is heated with 5 mL 0.1 M cadmium oleate solution at 60 °C under argon, to passivate the QD surface with


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cadmium ions. The most striking observation is the gradual disappearance of the 500 nm surface plasmon resonance as the reaction proceeds. In particular, Figure 5a shows the gradual decrease and the eventual disappearance of A2 over 77 minutes of the cadmium ion treatment. As shown in the Figure 5a inset, the amplitude of this feature drops roughly linearly as a function of the time for which the cadmium ion treatment is performed. The diminished amplitude of the plasmon resonance is correlated with the buildup of amplitude of the Cadmium signal in X-ray Photoemission (XPS) measurements (Figure S2). As previously reported for the higher temperature reactions, this treatment ultimately leads to the deposition of a CuFeS2-CdS alloy layer on the QD surface that is evidenced for example in the XRD pattern shown in Figure S3 supplementary information.

Figure 5. (a) Absorption spectrum of the CuFeS2 QDs heated with cadmium oleate at different time intervals. The variation in the amplitude of the A2 feature with reaction time is shown in the inset. (b) The conductance and photo response of CuFeS2 QDs increases with successive cadmium treatments. The dark I-V characteristics of CuFeS2 is shown in the inset. (c) Plot of ∆I against time. The light source is switched on at 0 minutes and switched off after 5 minutes.


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The strong importance of surface passivation on the properties of CuFeS2 films is further evidenced in conductance data. In these measurements, a film of CuFeS2 QDs was drop-cast on interdigitated platinum electrodes. The interdigitated electrodes consist of two platinum electrodes with digits placed at a distance of 10 µm. The individual digits are also 10 µm wide. The dimensions of the conductive area of the interdigitated electrode is 0.5 mm × 0.65 mm. Ascast films show a large dark current that is consistent with earlier reports of low sheet resistance of CuFeS2 structures.16, 48 The inset in Figure 5b shows the dark I-V characteristics of CuFeS2 quantum dot films. It is evident from the figure that the films are conductive and show ohmic behavior even without photoexcitation. This clearly indicates the presence of free charge carriers in the material. In addition, we observe the films to be only mildly sensitive to the presence of light. Upon illumination with a 14 W white light source coupled to a 550 nm long pass red filter, the films showed an increase in the current by 0.3 nA while the dark current is 95 nA. These measurements were done at a 1 V applied bias. The photoresponse is quantified as ∆I, which is the difference between the current value in dark and the current value upon illumination with the light source. We further developed a room temperature variant of the passivation process that enables us to perform a partial surface passivation of the CuFeS2 QDs with CdS. Briefly, we dipped a CuFeS2 film having OD of 0.64 at 2.5 eV in a solution of cadmium salt. The cadmium salt solution was prepared by dissolving 0.7 g cadmium acetate dihydrate in 1 mL butylamine and 4 mL ethanol. The progressive surface treatments lead to an increase in dark current of the material. More importantly, as shown in Figure 5b, the photoresponse of the film also increased after the first treatment. The variation of ∆I as a function of time is shown in Figure 5c. It is apparent that the treatment does not alter the temporal characteristics of the film optical response. The appearance of photoresponse indicates that improved surface passivation increases


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the semiconducting character of films. Collectively, these data are consistent with the interpretation of the A2 feature as a plasmon resonance that is suppressed upon passivation of the CuFeS2 QD surface. Also, the fact that CuFeS2 is a conductive material which develops a photoresponse after surface passivation suggests that the material is a potential candidate for the fabrication of photodetectors. In conclusion, we observed several peculiarities of CuFeS2 QDs that distinguish these from other I-III-VI2 semiconductor QDs. Most notably, these materials exhibit a LSPR in the 2.4 eV region of the spectrum. The energy of this resonance is observed to be sensitive to the dielectric environment of the QDs, making it interesting for sensing applications. The presence of free carriers is further evidenced in the cooling dynamics of CuFeS2 QDs as well as in the absence of a measurable photocurrent in these materials. We further find that presence of free carriers in these materials is linked to the surface passivation. In particular, it is seen that improved surface passivation can be achieved by the use of a room temperature cadmium ion treatment leading to the emergence of a photocurrent in CuFeS2 QD films. Our results thus consist with as prepared CuFeS2 QDs being suited for applications such as thermoelectrics and conductive layers. In contrast, optoelectronic applications that rely on the excitonic character of these materials require an additional post-processing step to improve QD passivation.

AUTHOR INFORMATION Corresponding Author * [email protected]


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Present Addresses ‡

Department of Applied Physics, Hong Kong PolyU, Hung Hom, Hong Kong

Notes The authors declare no competing financial interest. ASSOCIATED CONTENT Supporting Information Detailed synthetic procedure and characterization. XRD and TEM images of pure CuFeS2 and Cadmium treated sample XRD and XPS and the TEM images of the core/shell structures.

ACKNOWLEDGMENT We acknowledge Ms. Veena H. Iyer for help with certain parts of the study. We further acknowledge Mr. Dev K. Thapa for helpful discussions. AP acknowledges DST and ISRO-IISc STC for funding.

REFERENCES 1. Austin, I.; Goodman, C.; Pengelly, A. New Semiconductors with the Chalcopyrite Structure. J. Electrochem. Soc. 1956, 103, 609-610. 2. Teranishi, T. Magnetic and Electric Properties of Chalcopyrite. J. Phys. Soc. Jpn. 1961, 16, 1881-1887. 3. Khan, S.; Hesham, A. E.-L.; Qiao, M.; Rehman, S.; He, J.-Z. Effects of Cd and Pb on Soil Microbial Community Structure and Activities. Environ. Sci. Poll. Research 2010, 17, 288296. 4. Babayigit, A.; Duy Thanh, D.; Ethirajan, A.; Manca, J.; Muller, M.; Boyen, H.-G.; Conings, B. Assessing the Toxicity of Pb- and Sn-Based Perovskite Solar Cells in Model Organism Danio rerio. Sci. Rep. 2016, 6, 18721. 5. Kambara, T. Optical Properties of a Magnetic Semiconductor: Chalcopyrite CuFeS. II. Calculated Electronic Structures of CuGaS2:Fe and CuFeS2. J. Phys. Soc. Jpn. 1974, 36, 16251635.


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