Copper Iron Chalcogenide Semiconductor Nanocrystals in Energy and Optoelectronics Applications—State of the Art, Challenges, and Future Potential

I‐III‐VI2 semiconductor nanocrystals have been explored in countless optoelectronics and green energy applications. While indium‐based compounds are arguably the best‐known variants of these semiconductors and have been widely studied for their photophysical properties, other I‐III‐VI2 semiconductors are emerging as serious competitors. This review focuses on the current state of the art and recent progress made in the research and technology of one of the most promising competitors to indium‐based I‐III‐VI2 semiconductor nanocrystals, CuFeS2 nanocrystals. CuFeS2 is a promising alternative to indium‐based systems, mostly because many properties are competitive and because iron is much more abundant than indium. Replacing In(III) with Fe(III) would thus significantly alleviate the issue of raw material availability. The article highlights new synthesis approaches and summarizes and discusses advanced optical properties including surface plasmon resonance and luminescence properties. Moreover, potential applications in thermoelectric, photodetection, photothermal, and photovoltaics are illustrated and discussed. Finally, future perspectives for materials development, upscaling, and application in new processes and devices, such as self‐assembly, patterning, or plasmonic catalysis, are presented as well.


Introduction
During the last two decades, I-III-VI 2 semiconductor nanocrystals (NCs) have made significant progress toward

Synthesis
CuFeS 2 is composed of earth-abundant elements but the exact nanocrystalline form of this material was unknown for a long time. Due to various stable phases of both iron and copperbased sulfide materials, the synthesis of a single-phase copper iron sulfide was challenging. [44][45][46][47] In 1991, Silvester et al. reported the first hydrothermal synthesis of CuFeS 2 NCs with a particle size of 5-9 nm using iron oxide particles as an iron source. [48] The material was not as stable as those synthesized by other colloidal methods, which were reported later.
In 2010 Wang et al. and in 2012 Liang et al. reported the synthesis of CuFeS 2 NCs using colloid chemistry approaches. [49,50] Wang et al. synthesized different shapes of CuFeS 2 NCs and correlated their optical absorption behavior with the particle morphology. Liang et al. discussed the correlation between the nanoscale crystal structure and the thermoelectric performance. [50] In 2016, CuFeS 2 NC synthesis gained momentum via the development of reproducible hot injection synthesis routes producing NCs with interesting structural and optical properties. [51][52][53][54][55] Gabka et al. reported a new one pot colloidal high temperature synthesis of CuFeS 2 with different Cu/Fe ratios. [52] The same article also analyzes the different possible crystal phases in the resulting CuFeS 2 NCs like chalcopyrite or bornite. Figure 2a shows a typical powder X-ray diffraction (XRD) of a pure chalcopyrite phase of CuFeS 2 NCs, made by hot injection. Chalcopyrite is the thermodynamically most stable phase of these NCs and has therefore been studied for some time. Also, it has a perfect 1:1 Cu:Fe ratios in its stoichiometric composition.
Around the same time Ghosh et al. reported an alternative hot injection route for CuFeS 2 NCs. [53] Figure 2b shows the corresponding scanning transmission electron microscopy (STEM) images and the corresponding elemental maps obtained from scanning transmission electron microscopyenergy dispersive X-ray spectroscopy (STEM-EDX). These data confirm that all elements (Cu, Fe, S) are homogeneously distributed throughout the NCs, consistent with the crystallographic data. The same authors have also calculated the electronic band structure and analyzed the ultrafast electron dynamics of the intermediate bands. Bhattacharyya et al. reported a simplified colloidal hot injection synthesis for CuFeS 2 quantum dots (QDs) with tunable absorption from the visible to the infrared region [54] and reported effective mass and optical cross sections of the NCs. This is important for photovoltaic applications. The same authors also showed that the these NCs also exhibit a tunable strong luminescence with a core/shell NC architecture.
One of the key questions in NC design is how NC nucleation and growth can be controlled when more complex phases and multicomponent compounds are the synthetic goal. For example, Figure 2c,d shows XRD and size evolution data versus reaction time and temperature during NC synthesis. In all the aforementioned reports, there is one common aspect, that is unique and different from the traditional CuInS 2 NCs synthesis: the addition of reactive sulfur precursors was done after a heating-up phase and is not present in the initial reaction mixture. First, all metal precursors were dissolved in appropriate solvents and then the corresponding ligand was added. With the elevation of temperature, when a homogeneous solution of metal salt and stabilizer is achieved, a reactive sulfur precursor is added slowly to have an effective control over the nucleation process, which in turn leads to a single phase material.
One of the most interesting observations made in some of these studies is the fact that some of the NCs exhibit a surface plasmon, a feature that is commonly known from metallic nanoparticles but is not well explored in metal chalcogenides.  II-VI semiconductors where yellow and blue spheres denote the metal and chalcogens, respectively. b) Chalcopyrite structure where black, green, and yellow spheres denote the copper, iron, and sulfur. c) Unit cell of CuFeS 2 with antiferromagnetic ordering of Fe 3+ ions (blue spheres). Magnetic moment depiction (blue arrows) only applies to (c). Reproduced with permission. [43] Copyright 2015, American Chemical Society.  [54] Copyright 2016, American Chemical Society. b) STEM-EDX elemental maps of CuFeS 2 NCs. Reproduced with permission. [53] Copyright 2016, American Chemical Society. c) XRD patterns showing the crystal phase evolution during CuFeS 2 nanoparticle synthesis at different times and temperatures. d) Variation of particle sizes versus growth temperature and reaction time. Reproduced with permission. [62] Copyright 2021, American Chemical Society.

Surface Plasmon
Besides their different crystal structures (often resulting from different synthetic pathways), CuFeS 2 NCs also have fascinating optical properties which are highly significant for future optoelectronic applications. Figure 3a shows that CuFeS 2 NCs dispersed in organic solvents such as hexane show a characteristic purple color, and that CuFeS 2 NC films exhibit a golden luster. [55] Although the bulk bandgap of CuFeS 2 is 0.52 eV, the bandgap can be tuned from 2.5 to 0.53 eV in the quantum confined regime. [54,56] This tunability from visible to mid-infrared is highly attractive for the fabrication of optoelectronic devices. Moreover, like other I-III-VI 2 semiconductors, CuFeS 2 NCs exhibit optical absorption spectra with poorly defined spectral features. Similar to other I-III-VI 2 compounds, these optical properties are attributed to the weakening of selection rules of the optical absorption in the NCs which is caused by the mixed symmetry of the band edges. Likely, this is due to the fact that the Fe-d levels also participate in the band edge electronic structure. [53,55] Figure 3b shows an example of the optical absorption of CuFeS 2 NCs. Most importantly, this particular material has a surface plasmon resonance (SPR) [53,55] in the visible region at 500 nm, which is quite unique compared to other plasmonic semiconductors, such as Cu 2−x S [57,58] or Cu 2−x Se, [59][60][61] which have a plasmon resonance in the near IR regime. Interestingly, the position of this SPR feature is constant and does not change with differing Cu/Fe ratios or with the crystal structure of the NCs (e.g., chalcopyrite or bornite, Figure 3c). [62][63][64][65] Figure 3d shows the SPR absorbance of CuFeS 2 NCs at different growth times (and hence different particle sizes). These data demonstrate that the absorption band at 500 nm does not shift with changes of the particle size. This also illustrates that the plasmon in CuFeS 2 NCs is very robust. Moreover, even alloy materials based on CuFeS 2 exhibit a similar SPR band. For example, CuFeS x Se 2−x NCs have an SPR band at 408 nm ( Figure 3e) [66] and CuAl x Fe 1−x S 2 at 480 nm ( Figure 3f). [67] Further studies also indicate that the plasmon in these materials is quite stable and inert to different stoichiometric changes in the NCs. Very recently, Cu 3 VS 4 (sulvanite) NCs have shown similar but multiple SPR bands in the visible range. All these reports further broaden the appeal of nonmetallic plasmonic particles. [68,69] To elucidate and quantify the origin of the SPR-like features, various groups have studied the electronic structure of these materials. From these reports, it is quite clear that the origin of these SPR-like features is the Fe(III) ions, regardless of the band structure of the specific material under study. However, three different aspects of the band structures have been put forward as the cause of the SPR.  CuFeS 2 nanoparticle deposited as films (right) exhibit a golden luster. Reproduced with permission. [55] Copyright 2018, American Chemical Society. b) Typical NIR absorption spectra of CuFeS 2 nanoparticles. Reproduced with permission. [55] Copyright 2018, American Chemical Society. c) Correlation of absorption spectra and XRD patterns of NCs with different Cu/Fe ratios. Reproduced with permission. [52] Copyright 2016, American Chemical Society. d) Variation of 500 nm plasmon-like feature and bandgap with different growth times. Reproduced with permission. [55] Copyright 2016, American Chemical Society. e) Variation of a plasmon-like feature in the presence of selenium. Reproduced with permission. [66] Copyright 2019, The Royal Society of Chemistry. f) Absorption spectra of CuAl x Fe 1−x S 2 nanoparticles versus the Al/Fe ratios. Reproduced with permission. [67] Copyright 2016, American Chemical Society.
First, Ghosh et al. and Gaspari et al. showed that the origin of the SPR is an intermediate band position (Figure 4a). [70] These authors simulated the optical cross section of the material which exactly matches with experimental observation. Moreover, the same study found a shift in the resonance peak with a set of different dielectric media (Figure 4b,c).
Second, Sugathan et al. showed the presence of mobile carriers in the as-prepared NCs, positing a plasmon resonance as a possible origin of this feature, mostly because the study did not detect any intermediate band in a density of states (DOS) calculation ( Figure 4d). The same authors have also shown that the SPR like feature shifts with solvents of different refractive index (Figure 4e,f). [55] Third, Yao et al. reported that the SPR can be controlled by controlling the stoichiometry of iron atoms. [62] Finally, Lee et al. have observed a very similar SPR-like feature for AgFeS 2 NCs, [71] but both these effects and the variation of iron stoichiometry are not currently fully understood.

Luminescence
In general, I-III-VI 2 semiconductor NCs are emissive in nature. However, pure NCs often suffer from low quantum yields (QY) due to surface defects, which basically take part in nonradiative recombination processes. [72][73][74] For example, CuInS 2 NCs show a defect-mediated tunable broad emission with 3 to 10% QY. The origin of this luminescence is the recombination of conduction band electrons and copper mediated intragap defect states. The poor QY is greatly improved when a shell or gradient alloy structure with CdS or ZnS is added. This process often results in QYs in the range of ca. 90%. [75] A very similar behavior has been observed for CuFeS 2 NCs. The pure NC is nonemissive. This is due to free carriers as well as trapped surface states. [54] However, upon addition of a suitable shell, CuFeS 2 NCs become highly luminescent, very much like CuInS 2 . For example, Bhattacharyya et al. have shown that CdS shells can significantly enhance the emission of CuFeS 2 . [54] In some cases, very small CuFeS 2 NCs have QYs over 90% and the emission maxima can be tuned from the visible to near IR with different core sizes (Figure 5a).
Alike to the above examples, the one pot synthesis of Cd-Cu-Fe-S alloy NCs produces NCs with a bright infrared emission with 50% QY. [76] Yan et al. also reported that pure CuFeS 2 NCs are nonemissive but will emit in the near infrared region (NIR, 840 nm) with 52% QY upon addition of a ZnS shell ( Figure 5b). [77] Recently, Wu et al. have revealed a similar phenomenon for Cu 3 VS 4 NCs, where the QYs increased from 1% to 10% with a long radiative lifetime after adding a CdS shell. [78,79] Like other I-III-VI 2 semiconductor NCs, CuFeS 2 based core/ shell NCs also show long defect-mediated emission kinetics. Regardless of the shell (CdS or ZnS), CuFeS 2 based-core shell NCs have lifetimes on the microsecond time scale. [54,77] Figure 5c illustrates the variation of lifetimes with different bandgaps of the core material (CuFeS 2 NCs) and Figure 5d shows the lifetime of CuFeS 2 /ZnS core/shell NCs. The longest lifetime reported for CuFeS 2 based core/shell NCs is 10 µs. This is one order of magnitude longer than those observed for traditional ternary sulfide NCs like CuInS 2 . Currently, there is no clear explanation for this phenomenon, but it could be due to the large shell thickness of CdS or ZnS, possibly because shell synthesis remains a challenge to date and uniform shells are hard to achieve. One could speculate that there is a possibility of Fe leaching, which would then adversely affect the structural and hence optical properties. Increasing the shell thickness to avoid this leaching process will, however, have other effects limiting the liftetimes as well.

Ultrafast Dynamics
Ultrafast spectroscopies provide highly useful insights into the carrier dynamics of the materials. Ghosh et al. investigated the ultrafast transient absorption dynamics in CuFeS 2 NCs over the ≈400-700 nm spectral window. [53] The authors observed multiexponential transients corresponding to 20-50 ps time scale as well as a significantly slower component which is hundreds of ps long. The faster components were attributed to the intraband relaxation of photoexcited carriers in an intermediate band while the slower transient was interpreted as an interband relaxation.
Sugathan et al. probed the dynamics of both CuFeS 2 NCs as well as some corresponding core/shell materials. [55] They observed multiexponential dynamics in the system with bleach decays of 0.45 ps and 10.5 ps, respectively, when the system was pumped at a fluence that generated 0.4 excitons per NC. Since multiexciton excitation was not expected at these fluences, the authors interpreted these transients in the context of carrier thermalization in a doped system. In particular, the 0.45 ps transient was assigned to an electron-lattice coupling and the 10.5 ps transient was assigned to lattice cooling, as is expected from a plasmon resonance.
Sugathan et al. further studied the ultrafast characteristics of core/shell NCs as well. In particular they observed the absence of such dynamic features in the 2-2.5 eV region of the spectrum in the case of core/shell NCs. Instead, they observed a band edge bleach feature in CuFeS 2 /CdS core/shell NCs with a buildup and dynamics similar to CuInS 2 /CdS NCs. [54,55] These observations were taken to suggest a role of the CuFeS 2 NC surface in the observed dynamics. Moreover, these data were also taken as an indication that the band structure of the CuFeS 2 core does not play a role in the origin of transients in the 2-2.5 eV region of the spectrum, and, as a result, does not play a role in the SPR. As a result, Sugathan et al. inferred the additional lack of role of band structure in the ≈2.45 eV absorption feature.   [54] Copyright 2016, American Chemical Society. b) PL and absorbance of CuFeS 2 /ZnS nanoparticles. Reproduced with permission. [77] Copyright, 2019 The Royal Society of Chemistry. c) Emission kinetics of CuFeS 2 /CdS nanoparticles with different emission maxima. Reproduced with permission. [54] Copyright, 2016 American Chemical Society. d) NIR laser excitation images of CuFeS 2 and CuFeS 2 /ZnS nanoparticles and the emission kinetics of CuFeS 2 /ZnS. Reproduced with permission. [77] Copyright 2019, The Royal Society of Chemistry. materials. Correspondingly, these and other applications could be an attractive field of use for the future. As a result, the following sections will discuss the current state of the art and the potential for thermoelectric devices, photodetectors, and photovoltaics using CuFeS 2 NCs.

Thermoelectrics
Thermoelectric (TE) materials are a promising alternative energy source because TE materials can convert waste heat into electrical energy. [80][81][82] The efficiency of a thermoelectric material is defined by the dimensionless thermoelectric figure of merit, ZT (ZT = σS 2 T/κ), where S is the Seebeck coefficient, σ and κ are the electrical and thermal conductivities, respectively, and T is the average temperature of the material. TE materials can effectively exploit any heat difference and are therefore highly versatile in terms of their placement and position of use.
In spite of the apparent simplicity, there are a few pitfalls with TE materials. One of the key challenges is the fact that the best-known TE materials are made of toxic, rare, and expensive chemicals or from materials where the TE phase is unstable in the most interesting temperature range of ca. 350-650 K. [83] For example, tellurium, which is one of the main constituents of TE materials has an abundance of only 0.001 ppm in the earth crust and is toxic. There is thus a clear need to replace Te for the construction of TE devices and setups.
Indeed, very recently, copper chalcogenides have gained tremendous attention as an alternative because these materials are mostly made of earth abundant elements and many of these materials have stable solid-state structures and crystal phases. [84,85] Among all possible variations, CuFeS 2 is one of the most promising candidates for thermoelectric applications. [86,[87][88][89] Besides their availability, these ternary sulfides have an interesting structural feature. CuFeS 2 is a tetrahedrally bonded semiconductor with an elemental distribution of 1:1:2 (Cu:Fe:S). As a consequence, two different metal-sulfur (M-S) bonds form in the same tetrahedral coordinate where the Cu-S bond length is 2.198 Å and the S-Fe bonding length is 2.388 Å. As a result, the CuFeS 2 lattice forms an inherently distorted structure, which promotes the scattering of phonons (i.e., reduces the movement of phonons in the lattice) and therefore decreases the thermal conductivities. (Figure 6a). [90,91] In 2015 Mori et al. reported a high thermoelectric power factor with enhanced electron-magnon scattering of a naturally occurring Cu 1+x Fe 1−x S 2 mineral with antiferromagnetic nature. [92] In 2017 Tang   Reproduced with permission. [88] Copyright 2021, American Chemical Society. b) XRD pattern of zinc-doped CuFeS 2 with different Zn fractions. c) Corresponding figure of merit of the compound Cu 1−x Zn x FeS 2 (x = 0-0.1) at different temperatures. b,c) Reproduced with permission. [93] Copyright 2017, Wiley-VCH. d) Temperature dependent lattice thermal conductivity of CuFeS 2 with different metal substitution. e) Comparison between temperature dependent lattice thermal conductivity of CuFeS 2 with cubic and tetragonal phases. f) Comparison of thermal conductivities of some established TE materials. d, e, f) Reproduced with permission. [93] Copyright 2021, American Chemical Society.
concentrations which enhances the electric conductivities of these materials (Figure 6b,c). Finally, the same authors also showed a ZT value of 0.26 at 630 K with Cu 0.92 Zn 0.08 FeS 2 compositions. This is a more than 80% enhancement compared to CuFeS 2 pristine samples. [93] The substitution of Zn, Cd, Ag, or In into the chalcopyrite lattice can enhance the electrical conductivities, which is desirable. However, the incorporation of these additional elements does not lead to a significantly lower lattice thermal conductivity, which is also desired for good TE materials. (Figure 6d).
In a cubic geometry, however, the Cu-S bond length stretches from 2.198 to 2.292 Å, and the Fe-S bond length shortens from 2.388 to 2.292 Å. A change of the tetrahedral coordination to an octahedral coordination in the CuFeS 2 lattice would significantly decrease the thermal conductivity but cubic structures still show better overall TE performance (Figure 6e,f). As a result of the high structural flexibility, the high degree of possible substitution, and the corresponding adaptability of the properties for TE applications, there are currently more than forty reports of TE properties of bulk CuFeS 2 , both in their chalcopyrite and bornite phases, see, e.g., refs. [86][87][88][89][90][91][92][93].
In contrast to the fairly large body of work on bulk CuFeS 2 TE materials, there are only few reports on nanostructured CuFeS 2 TE materials. This is interesting because once these materials are in a nanoscale dimension, a number of properties change. For example, due to quantum confinement, the electronic den-sity of states (DOS) is altered which can, in turn, directly affect the Seebeck coefficient. At the same time, the number of surface and interface defects increases with smaller particle size (i.e., with larger surface area) in comparison to bulk materials. This can alter both the heat and electric transport in these nanostructured materials versus their bulk counterparts.
In 2012, Liang et al. reported the TE response of CuFeS 2 nanoparticles from 300 to 500 K (Figure 7a). The study found a power factor of 0.264 at 500 K; this is 77 times higher than values observed for bulk chalcopyrite and reproducible ZT values were observed with different batches of material (Figure 7b,c). [50] In 2017, Gabka et al. reported a gram-scale synthesis of CuFeS 2 nanoparticles and their TE properties after producing a hot-pressed pellet form the chalcopyrite NCs, [94] (Figure 7d). Furthermore, the same authors also investigated the role of ligands and different Cu:Fe stoichiometries in the TE performance (Figure 7e).
In 2018, Vaure et al. observed very similar nanoscale thermoelectric performances. In addition, they have also studied various dopant effects like selenide (Se 2− ) anion doping or Sn nanoparticle doping. In both cases, there is an enhancement of the overall power factor although the reasons for this enhancement are not exactly clear. For example, pristine CuFeS 2 has a ZT value of 0.08 at 400 K but upon addition of 3 wt% Sn nanoparticles, ZT increased to 0.14 ( Figure 7f,g). [95] Overall, it currently appears that CuFeS 2 nanoparticles are promising candidates for high ZT (≈1) materials at room temperature.  However, there are currently better materials available and further research is necessary to fully exploit the potential of CuFeS 2 and related TE materials.

Photodetectors
Photoactive materials can emit and harvest light in various optoelectronic devices such as photodetectors. While photodetection in the visible range has been established for a while, [96,97] the detection of infrared (IR) photons is still a challenge because both CO 2 and H 2 O have strong absorption bands in this region and conventional silicon detectors cannot detect signals beyond 800 nm. To resolve these issues, specialized detectors have been designed for various spectral regions like near IR (NIR, 900-2500 nm), mid IR (MIR, 3000-5000 nm), and far IR (FIR, 8000-12000 nm). Well known IR photodetector materials include single crystalline HgCdTe, [98,99] InGaAs, [100,101] and InSb [102] as well as epitaxial superlattice structures. [103] Besides the more traditional single crystal materials, colloidal semiconductors have gained attention as IR photodetector materials. The high interest is due to a broad absorption spectrum, high absorption coefficients, size-tunable optoelectronic properties, and easy solution processability of colloidal NCs. [104,105] In 2012 Konstantatos et al. have reported a hybrid graphene/ PbS NC phototransistor exhibiting a very high photoconductive gain of ≈10 8 and a detectivity up to 7 × 10 13 Jones. [106,107] Other materials have been reported as well, for example a dual band infrared detector based on mercury chalcogenide semiconductor NCs. [108] Therefore, it is clear that semiconductor NCs are attractive for the construction of photodetectors. However, most of these detector materials contain significant amounts of hazardous heavy metals like Cd, Pb, or Hg. Therefore, it is highly desirable to replace these components with nontoxic, benign, and sustainable constituents.
Copper iron chalcogenides (both CuFeS 2 and CuFeSe 2 ) could be a promising alternative to heavy metal-based compounds. Both materials have an IR bandgap with high optical crosssection through the visible to MIR window. [109] Consequently, in 2019, Sugathan et al. reported a broad photodetection window from the visible to the mid IR region with FeS 2 /Si NC/bulk heterojunctions (Figure 8a,b). [110] This architecture produced a strong photocurrent response from 460 to 2200 nm under ambient conditions (Figure 8c). The highest responsivity and detectivity was 4.68 mA W −1 and 5.29 × 10 9 Jones at 1900 nm (Figure 8d). [110] In 2020, Kumar et al. reported a bilayer p-n junction made of n-ZnO and p-CuFeS 2 NCs for IR photodetection (Figure 8e,f) and [glass/CuFeS 2 /Ag] single layer photodetector (Figure 8g,h). These authors claimed a photodetectivity above 10 12 Jones in the visible−NIR region under 10 V external bias. [111] In 2021, Kumar et al. also showed a similar IR photodetection with CuFeSe 2 NCs in a p-n junction geometry on top of an Si substrate. [112]

Photothermal Properties
Photothermal therapy (PTT) is a noninvasive therapeutic technique where nanomaterials are transported into tumor cells [113] and convert absorbed photon energy to heat that will eventually   [110] Copyright 2020, Wiley-VCH. e,f) Current versus voltage characteristics of a (glass/ZnO/Ag/ CuFeS 2 ) bilayer heterojunction photodetector with schematic device architecture and band diagram. g,h) Current versus voltage characteristics of a (glass/CuFeS 2 /Ag) single layer photodetector with schematic device architecture and band diagram. e-h) Reproduced with permission. [111] Copyright 2020, American Chemical Society. kill the cancer cells. Materials which absorb NIR light (650-1200 nm) are the best candidates for these applications because NIR light is capable of deep tissue penetration with low scattering and will not damage the surrounding tissue. [114,115] Due to the high importance of PTT, a vast number of inorganic materials such as metal nanoparticles, [116] IR absorbing metal complexes, carbon nanotubes, [117] graphene, [118] and organic materials like IR dyes [115] have been used for this purpose.
Among the metal nanostructures, gold nanorods have been used extensively as a photothermal agent, [119,120] but their major drawback is the fact that to obtain an efficient PTT performance, the aspect ratio of nanorods should be high. [121,122] This, however, causes issues with transport to the cells of interest. Therefore, along with the high photothermal efficiencies, the ideal photothermal materials should be small and ideally isotropic so that they can easily be transported to the cells and then again be released through the renal clearance route. Generally, particles with hydrodynamic diameters below 5 nm can easily pass through the kidney but not larger nanoparticles. [121][122][123][124] In spite of this, most current PTT agents still have an average size much more than 5 nm [123,124] and there is thus a clear need for new materials that allow for a further size reduction while maintaining the existing advantageous properties. Among other candidates, copper chalcogenides have therefore been evaluated as future PTT materials and Li et al. first reported copper chalcogenide nanoparticles for photothermal cancer ablation therapy. [125,126] More recently, copper iron chalcogenides have gained profound interest in this regard. Being a narrow bandgap IR semiconductor with a strong plasmon-like feature and the fact that they are based on earth abundant constituents, have made these materials prime candidates for PTT. In 2016, Ghosh et al. for the first time reported the photothermal conversion of CuFeS 2 with 49% efficiency. Using a 808 nm laser excitation they observed a photothermal conversion efficiency that was more than one order of magnitude higher than that of standard photosensitizer photothermal dyes. [53] Figure 9a shows thermal images of CuFeS 2 NCs dispersed in water in different concentrations with 808 nm laser excitations for various times. Ding et al. have described cis-platin decorated CuFeS 2 nanoplatelets for thermoacoustic imaging of cancer cells (Figure 9b) and a PTT experiment with 30.1% conversion. [127] Girma et al. have applied a similar concept using CuFeS 2 nanoparticles for tumor-targeted chemotherapy and PTT using CuFeS 2 nanoparticles decorated with hyaluronic acid and cis-platin resulting in a carrier with an ultimate photothermal conversion efficiency of 72.14%. [128] Figure 9c,d show the temperature evolution and corresponding on-off cycle of CuFeS 2 NCs in aqueous medium with NIR laser excitation. CuFeSe 2 NCs have also been used for PTT. [129,130]   concentrations, exposed to the same power 808 nm laser light for different times after injection. Reproduced with permission. [127] Copyright 2017, The Royal Society of Chemistry. c,d) Temperature evolution and corresponding on-off cycle of CuFeS 2 NCs in aqueous medium with laser excitation. Reproduced with permission. [128] Copyright 2018, American Chemical Society. e) CT images (upper) and X-ray attenuation of intensity in Hounsfield units (HU) of CuFeSe 2 NCs as a function of NCs concentration in contrast to iopromide (lower). f) Illustration of multimodal imaging (PAI, CT, SPECT/CT, MRI) and photothermal therapy of cancer using CuFeSe 2 NCs. e,f) Reproduced with permission. [131] Copyright 2017, American Chemical Society. intensity (Hounsfield units, HU) as a function of NC concentration in contrast to iopromide. These data clearly show that CuFeSe 2 NCs exhibit almost one order of magnitude better performance than standard iopromide in terms of X-ray attenuation. Besides PTT, the same particles also act as a nanotheranostic agents for multimodal imaging like photoacoustic imaging (PAI), magnetic resonance imaging (MRI), and computed tomography (CT) imaging (Figure 9f). [131] For example, Dang et al. have reported bioactive glass scaffolds functionalized with CuFeSe 2 NCs by combining 3D printing with solvothermal reaction process. These scaffolds can be used for PTT tumor therapy and bone reconstruction. [132]

Photovoltaics
I-III-VI 2 semiconductors have been widely investigated for photovoltaic applications with various cell architectures. Due to the ease of NC synthesis and relatively low cost, these semiconductor NC inks have become an efficient approach to substitute high cost vacuum-based deposition processes that are currently used for the fabrication of thin film solar cells. For example, a thin film solar cell made of copper indium gallium selenide (Cu(In 1−x Ga x )Se 2 ) has a certified efficiency of 23.4%, while a Zn-Cu-In-S (ZCIS) QD ink solar cell has 16% efficiency. That is, the efficiency of the QD solar cell is still lower, but the production cost is lower as well and, additionally, QD inks enable the fabrication of flexible solar cells. In spite of the apparent advantages of QD ink-based solar cells, they have not been commercialized yet. In part this is due to the use of relatively rare and expensive indium and gallium.
Like other I-III-VI 2 semiconductors, copper iron chalcogenides have also been considered as an alternative material for photovoltaics. However, due to the plasmonic character associated with the 3d electrons of iron they have always shown a metallic character in conductivity measurements. However, when alloyed with aluminum or when a proper core shell structure is constructed, the photoresponsivity of these materials is drastically enhanced.
Li et al. have used CuFeS 2 NCs as a counter electrode in a dye-sensitized solar cell (DSSC) for the I 3 − reduction/oxidation cycle and reported a power conversion efficiency of 8.10%. This is similar to devices where a platinum counter electrode was used in the DSSC cell (Figure 10a).
In 2016, Zhang et al. studied CuFeS 2 NCs in a conducting polymer matrix. The monodisperse CuFeS 2 NCs exhibit a broad absorption and show magnetic characteristics which enable the coupling between optoelectronics and magnetoelectrics under external electric and magnetic fields (Figure 10b). [133] Wang et al. observed a very similar photoresponse in CuFeSe 2 NC-based films where the current of CuFeSe 2 films increased from 0.13 µA (dark) to 0.94 µA (under Xe lamp exposure); this corresponds to a 7.2-fold increase upon irradiation. [134,135] Adv. Optical Mater. 2023, 11, 2202411   Figure 10. Photovoltaic response of CuFeS 2 NCs. a) Photocurrent density-voltage (J-V) curves of the DSSCs using CuFeS 2 NCs and Pt as counter electrodes. Reproduced with permission. [135] Copyright 2016, The Royal Society of Chemistry. b) I-V curves of a CuFeSe 2 NC thin film device in the dark state and under Xe illumination, and corresponding time-dependent response of the photocurrent measured at ambient atmospheric conditions with 1.0 V bias voltage. Reproduced with permission. [133] Copyright 2015, American Chemical Society. c) CuFeS 2 NCs as hole transport layer in a CdTe solar cell. Reproduced with permission. [136] Copyright 2018, Cambridge University Press. d) Device architecture of CuAl 0.25 Fe 0.75 S 2 and CuFeS 2 based solar cells and e) corresponding J-V characteristics of the device. d,e) Reproduced with permission. [137] Copyright 2020, American Chemical Society.
In 2018, Bastola et al. demonstrated the potential of CuFeS 2 NCs as hole transport layer in CdTe thin film solar cells (Figure 10d). [136] In 2020 Naveena et al. demonstrated the first thin film solar cell made from CuAl 1−X Fe X S 2 NCs with an efficiency of 3.65 and 2.33% for the CuAl 0.25 Fe 0.75 S 2 and CuFeS 2 thin films, respectively (Figure 10e,f). [137] Moreover, Sekiya et al. showed that CuFeS 2 can be used as a thermally sensitized cell to convert heat into electricity directly. [138] 7. Future Outlook

Scalable Synthesis
Besides the fundamental challenges in nanomaterial synthesis, characterization, and property tuning, there is a further challenge that needs to be addressed: uniform production of large amounts of a specific nanomaterial. Controllable and reliable nanomaterial synthesis on a kilogram or even ton scale is a challenge, for multiple reasons: (1) unlike in classical organic (or better: molecular) synthesis, nanoparticles synthesis usually starts from homogeneous reaction mixtures (the precursors are dissolved in a solvent) but as the reaction proceeds, the mixture becomes heterogeneous because a second phase (the nuclei and then crystals) forms, which generates challenges with heat and mass transfer in larger volumes, (2) one pot reactors with large volumes produce different results than the same experiment conducted in smaller volumes; this is due to changes in the surface-to-volume ratio and to effects of stirring, which are not easily scalable, and (3) control over heat flow in large vessels is a challenge; this control is, however, important because nucleation and growth rates of QDs often critically depend on temperature.
As a result, there is a clear need to not only understand the fundamentals of nanomaterial growth in the laboratory, but also to develop scalable processes for a sustainable and controllable synthesis of larger amounts of QDs. The challenges beyond the scientific aspects have been covered in the literature. [139] In an interesting recent study, Jean et al. [140] have illustrated how the synthesis of QDs is not only a factor of cost of the actual precursors but how additional factors come into play as one compiles the necessary infrastructure that is needed, such as equipment cost, equipment depreciation, salaries of the workforce involved, possibly repairs and downtimes, etc. A newer study backs these data and also highlights the effects of solvent recycling and other aspects directly involving the environmental impact of the respective processes. [141] Environmental impact as a central issue has been brought up earlier, but will certainly gain more momentum in the future.
Overall, the aspects just discussed are not typically addressed in a research publication but both economic and environmental (safety) factors are crucial when truly attempting to cross the so-called "valley of death." [142] This is a zone or phase between invention/development and true production when most of the initial promising projects fail due to issues that are not related to the actual scientific problem but rather because some of the obstacles outlined above come into play.
We have recently elaborated on these issues in two articles [143,144] and on how these factors are not only an issue for the NCs discussed in the original study by Jean et al. but also for other high-tech materials or components. As such, making a few grams of NCs is clearly possible but there are challenges that must be overcome to reliably deliver substantial amounts on the order of at least kilograms. Therefore, the development of robust, reliable, and ideally scalable nanoparticle production processes will remain a challenge in the foreseeable future and should not be underestimated.

Patterning and Self-Assembly
Some nano-or microstructured materials are now produced on a large scale using both top-down and bottom-up methods. [145] Top-down strategies are based on the generation of a mask by site-selective ablation of material from a thin film deposited on a substrate. Photolithography or electron beam lithography, for example, are used for this purpose. The resulting nano-and/or microstructured layers serve as a mask for the subsequent deposition of the material that has the desired properties for the intended device. The deposition of this material can be accomplished by various techniques that have also been described for the production of thin films of copper iron chalcogenides. These techniques include, e.g., flash evaporation, [146] vacuum evaporation in combination with sulfurization, [147] and spray pyrolysis. [148,149] However, as far as we are aware, the fabrication of copper-iron chalcogenide films with nano-or micropatterns has hardly been reported in the literature. This is particularly surprising in view of the plasmonic properties of copper iron chalcogenide nanostructures. [55] The attractiveness of plasmonic nanomaterials is based on their SPR, which can occur in a wide range of wavelengths. The spectral position of the SPR is controlled by several parameters, including the choice of material, the size and morphology of the nanoparticles and, last but not least, the geometrical arrangement of the plasmonic nanoparticles. [150] In the latter case, the distance between the plasmonic nanoparticles is reduced to values equal to or smaller than the penetration depth of the SPR into the surrounding medium. As a result, the plasmons of two or more adjacent plasmonic nanoparticles can interact with each other and so-called plasmonic coupling occurs. [151] Plasmonic coupling leads to drastic changes in the SPR properties of the plasmonic nanoparticles involved and is fairly well understood theoretically. Advanced theoretical calculations based on solving, e.g., Maxwell's equations can predict these changes in SPR. [152] That is, by tailoring the fabrication of plasmonic nanomaterials with defined geometric arrangement of nanoparticles, targeted materials can be obtained for a variety of applications such as optical sensors, surface-enhanced spectroscopy, and catalysis.
In addition to the top-down approaches already described, bottom-up strategies are also exploited to obtain nanostructured plasmonic materials. Here, plasmonic nanoparticles are used as building blocks that self-assemble into ordered arrays or multilayers, i.e., 2D and 3D materials. [153] To support the selfassembly process, the surface of colloidal plasmonic nanoparticles is often modified with suitable ligands that either serve only as interparticle spacers to obtain ordered monolayers of nanoparticles or that can actually control the position of the plasmonic nanoparticles in the overall arrangement. Polymers are commonly used as spacers, such as thiol-terminated polystyrenes, [154] while, e.g., DNA origami is utilized to realize advanced plasmonic nanoparticle assemblies with highly specific 2D and 3D architectures. [155] In addition, chemically and topographically patterned substrates facilitate the formation of plasmonic nanoparticle assemblies with a high degree of order required for the fabrication of high-quality plasmonic devices. [156] Clearly, the current focus of such approaches is not on copper iron chalcogenides but in rather well-established metallic materials. However, as CuFeS 2 can exhibit a surface plasmon, all of these strategies could also be used to study the effects of geometric arrangement on the plasmonic properties of copper iron chalcogenide nanoparticle arrays. Plasmonic coupling in layers of copper iron chalcogenide nanoparticles may have already been observed, as is evidenced from a plasmon band shift in one example. [55] The phenomenon has, however, not been further researched. This leaves much room for further study both on smart tools and strategies for the assembly of copper iron chalcogenide (and other chalcogenide) nanoparticles, but also on the subject of understanding the details of the physicochemical effects that are then observed in these architectures.

Alternative Plasmonic Materials
Besides the challenges in upscaling and controlling the 2D and 3D architecture of copper iron chalcogenide NCs discussed so far, the plasmonic properties of CuFeS 2 make this material an interesting alternative to noble metals for plasmon-enhanced spectroscopy, most importantly for surface-enhanced Raman scattering (SERS). [157] SERS relies on the electromagnetic field enhancement close to the surface of a nanomaterial upon excitation of their SPR. Au and Ag are the most common materials used for plasmon-enhanced spectroscopy, however, alternative semiconducting materials have been explored recently. [158] Inorganic and organic semiconductors can exhibit considerable SERS signal enhancements, however, the enhancement is typically ascribed to a chemical mechanism that is based on charge transfer. [159] Materials such as CuS and CuSe have been used for SERS, but only in conjunction with noble metals [160] and the signal enhancement of these materials alone is low. Therefore, the demonstration of pronounced SPR features in CuFeS 2 [51][52][53][54][55] promises new avenues in the exploration of, e.g., SERS substrates.
Plasmonic materials do not only serve as passive enhancers of the electromagnetic field, but due to the possibility of generating hot charge carriers and heat upon SPR excitation, also chemical processes can be triggered, which is the basis of plasmonic chemistry. Due to the possibility to obtain chemical information at the surface of nanoparticles, SERS has often been used to demonstrate chemical transformations on the surface of plasmonic materials and to obtain kinetic information. [161,162] This new field of photocatalysis is very promising because visible light can be efficiently harnessed and converted to chemical energy. Very recently, CuFeS 2 has been applied as a plasmonic photocatalyst for the selective reduction of nitroarenes using solar energy as the only energy source. [163] Very high reaction rates have been demonstrated with unprec-edented turnover frequencies at high production rates and high cost-normalized rates.

Water Treatment
Besides the application of copper iron sulfide in plasmonic catalysis, other fields will likely also gain attention for these materials, for much of the same reasons: cheap precursors, simple synthesis, and high adaptability, tunable optical, electronic, and catalytic properties. Arguably the most prominent field besides optics and energy will be environment and sustainability. [164] For example, water treatment and remediation is among the absolute top priorities worldwide. The World Health Organization (WHO) has outlined the most pressing issues, such as the fact that in 2022, "over 2 billion people live in water-stressed countries" (https://www.who.int/news-room/ fact-sheets/detail/drinking-water) and this number is likely to grow in the future. As a result, countless methods have been developed for treating a multitude of contaminations in water bodies. [165][166][167][168][169] Nanotechnology is a prime candidate to provide solutions to some of the central issues such as decontamination, removal of bacteria, pesticides, endocrine disruptors etc. [170] A particular challenge is found in the field of micro pollutants or refractory pollutants, that is, pollutants that are harmful but very hard to remove due to their very low concentration in (surface) water. [171] Interestingly, however, there are no references to using copper iron sulfide for water treatment. This is intriguing, because there are numerous accounts of using other metal sulfides for water treatment, most notably heavy metal removal, [172][173][174][175] denitrification or for use in advanced oxidation processes to remove refractory organic contaminants. Clearly, the optical and photocatalytic properties of copper iron sulfide nanoparticles or thin (porous) films would render them interesting candidates for photocatalytic water treatment of organic or inorganic contaminants, possibly with better properties than the current materials. Tying back into the chapters above, also solar evaporators based on chalcopyrite plasmonic nanoparticles could be envisioned, but have not been realized so far.

Conclusions
CuFeS 2 NCs have tremendous attention from fields as different as photocatalysis, medical materials, and spectroscopy. This is due to the virtually unlimited possibility to tune the properties of the base material for a very specific application by way of chemical composition, particle size, or particle arrangement. Predominantly, CuFeS 2 -based NCs have been promoted as alternate green energy material. [176][177][178][179][180][181] Two main reasons are responsible for this interest. First, the NCs exhibit a plasmon like resonance at ca. 500 nm and a semiconductor with an SPR in the visible range is quite unknown or unexplored. Second, the NCs also show an infrared bandgap yet they are made from rather abundant materials. CuFeS 2 NC therefore combine a series of technologically and ecologically relevant aspects and this article therefore strives at compiling the information that is currently available on these highly interesting and versatile materials. The article also shows that we are just at the beginning of the journey and that likely a lot more is to come from these unique NCs.