Hybrid Chalcopyrite–Polymer Magnetoconducting Materials

Letter www.acsami.org

Hybrid Chalcopyrite−Polymer Magnetoconducting Materials Zhuolei Zhang, Beibei Xu, Lin Zhang, and Shenqiang Ren* Department of Mechanical Engineering, Temple University, Philadelphia, Pennsylvania 19122, United States S Supporting Information *

ABSTRACT: The search for emerging materials combining magnetic and semiconducting properties has attracted widespread interest in contemporary materials science. Chalcopyrite (CuFeS2), as an earth abundant and nontoxic chalcogenide compound in the I−III−VI2 family, is a promising class of such materials that exhibit unusual electrical, optical, and magnetic properties. However, its successful implementation largely depends on our ability to understand, control and manipulate their structural, transport and spin behavior. Here we show that solution processing monodispersed CuFeS2 quantum dots exhibit a strong coupling among optical, electronic, and magnetic degrees of freedom. The photoresponse and magnetoconductance of CuFeS2 quantum dots are realized under external stimuli. We further exploit a fast and efficient way to achieve an exceptionally large performance in a magneto-optoelectronic hybrid system consisting of magnetic semiconducting CuFeS2 and conducting polymer matrix. The results demonstrate a promising potential of magnetic semiconductor CuFeS2 in the field of spin electronics. KEYWORDS: nanocrystals, magnetism, optoelectronics, magnetic semiconductor, organic−inorganic hybrid materials eV, a large optical absorption coefficient (1 × 105 cm−1) and the Néel temperature of 823 K.29−31 However, in comparison to other chalcogenide compounds such as CuInS2 and AgInS2, the carrier concentration of magnetic CuFeS2 semiconductor is relatively high while the conductivity is unusually low, due to the strong coupling between charge carriers and their magnetic moments.32−35 The low conductivity of CuFeS2 is resulted from the existence of spin polarons induced by the surrounding magnetic moments to enable immobile magnetic domain, which becomes an obstacle for CuFeS2 to achieve power efficient electronics, energy harvesting and storage devices.36 One way to overcome charge-transport limitation is to introduce a hybrid material design by combining CuFeS2 with organic conducting polymer matrix, in which remarkably fast charge transfer is favored between high electron affinity CuFeS2 and low ionization potential conjugated polymers.25,37−39 Furthermore, the advent method of controlling CuFeS2 on the nanometer scale (quantum dots: QDs) could essentially open up new opportunities for the development of next-generation, low-cost, and flexible hybrid spin-optoelectronics. Therefore, in this study, we utilize colloidal nanosynthesis and solution assembly to create charge transfer junction between monodispered CuFeS2 magnetic semiconduct-

he emerging research area “spin electronics” has developed rapidly in the recent decades, which employ both spin degree of freedom of electrons and their charges.1−6 Within this context, a lot of pioneering studies have been done on disorder effects in nonmagnetic semiconductors and metals, as well as half-metals with 100% spin polarization at the Fermi level.7−11 The search for magnetic semiconductors - coexisting the magnetic and semiconducting properties−has evolved into an important field of modern material science. Within such materials, magnetism has been proven to be carrier-mediated, which could enable magnetic modification through charge manipulations.12−16 Therefore, the design of materials combining both magnetic and semiconducting properties turns out to be vital for the development of spin electronic devices with highly spin-polarized carriers and has motivated a continuous search for new materials.17−21 Chalcopyrite (CuFeS2), as an earth abundant, nontoxic chalcogenide compound in the I−III−VI2 family, exhibits unusual electrical, optical, and magnetic properties, which could find practical significance for photovoltaics, thermoelectrics, lithium-ion battery, and spintronics.22−28 As an antiferromagnetic semiconductor, CuFeS2 crystallizes in a tetragonal structure, whereas Cu and Fe atoms are coordinated with four neighboring sulfur ions and alternating Fe and Cu metal layers are separated by the sulfur layers. Bulk magnetic semiconducting CuFeS2 material exhibits a band gap of 0.5−0.6

T

© 2016 American Chemical Society

Received: March 18, 2016 Accepted: April 25, 2016 Published: April 25, 2016 11215

DOI: 10.1021/acsami.6b03362 ACS Appl. Mater. Interfaces 2016, 8, 11215−11220

Letter

ACS Applied Materials & Interfaces

Figure 1. (a) TEM images of the representative CuFeS2 nanocrystals. Inset: HRTEM image of a representative CuFeS2 particle. (b) XRD patterns of the CuFeS2 nanocrystals. The standard diffraction patterns of chalcopyrite bulk CuFeS2 included as reference. (c) UV−vis spectra of the representative CuFeS2 nanocrystals Insets show the Plot of (αhv)2 versus photon energy (hv) in the band edge region for determining the band gap. (d) M−H loop of the representative CuFeS2 nanocrystals.

Figure 2. (a) Typical characteristic schemes of the device for measurement of photoelectric and magnetoelectric property. (b) Current density−voltage (J−V) curves of CuFeS2 nanocrystals under dark and light illumination of 18 mW/cm2. (c) Photoresponse behavior of CuFeS2 QDs under external light illumination from 3 mW/cm2 to 18 mW/cm2. (d) Relationship of the photocurrent density with light intensity of the CuFeS2 nanocrystal-based device.

ing QDs and conjugated polyaniline (emeraldine base) with high interfacial area. Here, we report the controllable synthesis of CuFeS2 nanocrystals based on a wet chemical hot injection method. The particle size and shape of CuFeS2 QDs are examined using

transmission electron microscopy (TEM). As shown in Figure 1a, the monodispersed CuFeS2 nanocrystals exhibit the spherical shape with an average diameter of 8 nm. The high resolution TEM (HRTEM) image of CuFeS2 nanocrystal is shown in the insets, exhibiting clear lattice fringes with a spacing of d = 0.304 11216

DOI: 10.1021/acsami.6b03362 ACS Appl. Mater. Interfaces 2016, 8, 11215−11220

Letter

ACS Applied Materials & Interfaces

Figure 3. (a) Time-dependent current density change with and without a light density of 18 mW/cm2 under external magnetic field from 1000 to 4000 Oe. (b) MC with different applied magnetic fields under dark and under excitation of 18 mW/cm2. (c) Time-dependent current density change with and without an external magnetic field of 4000 Oe under various external electric fields. (d) Dark MC under various electric fields.

strongly influenced by external electric and magnetic field. Magnetic field effects on CuFeS2 device is investigated in Figure 3a, which shows magnetic field dependent current density with a bias electric field of 5 kV/cm. The magnetoconductance (MC = ΔJ/J0, and ΔJ refers to the relative current density change with and without a magnetic field) is plotted in Figure 3b, in which a positive magnetic field effect is shown. The presence of positive magnetic conductance effect can be understood as the result of localized magnetic moment scattering, originally reported by Toyozawa.40 The application of an external magnetic field orders the magnetic moments and reduces scattering, resulting in a increase in conductivity.41 The negative magnetoresistance is Δρ derived with the form: ρ = −A12 ln(1 + A 2 2 H2), where A1 and

nm, corresponding to the (112), crystallographic plane of the chalcopyrite phase. To identify the structure and phase purity of the samples, we further investigate the crystallographic properties by using powder X-ray diffraction (XRD), in which the CuFeS2 shows a tetragonal crystal unit cell matched with the JCPDS database (65−1573). Using Scherrer’s formula, the size of CuFeS2 particles are estimated around 8.5 nm, which is qualitatively consistent with the observation from the TEM measurement. The optical characterization of CuFeS2 QDs is performed by recording UV−Vis absorption spectra (Figure 1c), which exhibit a broad photoabsorption from blue to nearinfrared. The optical band gap of the CuFeS2 QDs is estimated about 1.08 eV (The inset of Figure 1c), indicating the potential of CuFeS2 nanocrystals as a good light absorber for energy harvesting. Furthermore, the magnetization hysteresis (M−H) loop of CuFeS2 QDs is shown to reveal its magnetic semiconducting behavior (Figure 1d), in which the CuFeS2 QDs show a room-temperature saturation magnetization (MS) and coercivity of 0.97 emu/g and 85 Oe, respectively. The CuFeS2 QDs with broad photoabsorption are investigated further for their optoelectronic properties, where the experimental scheme of solution processed thin film device is illustrated in Figure 2a. The current density−voltage (J−V) curves of CuFeS2 device are shown in Figure 2b under dark and light illumination of 18 mw/cm2, where the resistivity (ρ) is determined as 6.45 × 106 Ω m under dark. Figure 2c shows the photoresponse behavior of CuFeS2 QDs, where the current density dramatically changes with a repeatable and reversible on/ off ratio from 1.1 to 16.4% as the external light density from 3.1 to 20.2 mW/cm2. The relationship between photocurrent density and light intensity of the CuFeS2 QDs is shown in Figure 2d, which could be fitted by the linear relationship of J = J0 + AW, where J is the current density, J0 is the initial dark current density (0.61 mA/cm2), W is the light intensity, and A is a constant (0.0082). As a magnetic semiconductor material, CuFeS2 occurs strong coupling between the charge carriers and magnetic moments, and therefore, the current density of CuFeS2 QDs could be

A2 are related to various physical quantities associated with exchange interaction (details are shown in the Supporting Information). As external magnetic field H increases from 1000 to 4000 Oe, the magnetoconductance is increased from 0.32 to 3.79% because of the suppression of thermodynamic spin fluctuations, thus promoting negative magnetoresistance. To gain insight into the MC effect on magnetic semiconducting CuFeS2 QDs, we performed magnet resistance measurement to exhibit magnetic-field-negative resistance effect (Figure S1). Furthermore, the photoexcitation could improve the MC effect as shown in Figure 3b, where the resistance of CuFeS2 QDs decreases with external light illumination and external magnetic field. In addition, the current density is increased more than 3.5 times as increasing external electric field from 2 to 6 kV/cm because of the enhanced charge density by external charge injection. A positive magnetoconductance is observed with external electric field, in which the MC value achieves an optimum value of 4.45% under an electric field of 10 kV/cm. Such behavior could result from magnons interacting with drifting carriers, leading to the ordering of magnetic moments and further decreasing of the electric resistance. The strong coupling among optical, electric, and magnetic degrees of freedom in CuFeS2 QDs makes it a promising material for multifunctional applications. However, the intrinsic low 11217

DOI: 10.1021/acsami.6b03362 ACS Appl. Mater. Interfaces 2016, 8, 11215−11220

Letter

ACS Applied Materials & Interfaces

Figure 4. (a) UV−vis absorption spectrum of polyaniline, CuFeS2, and CuFeS2‑PANi complex. (b) Current density−voltage (J−V) curve of CuFeS2 nanocrystals and CuFeS2‑PANi complex-based devices. (c) Time-dependent current density change of CuFeS2 nanocrystals and CuFeS2‑PANi-based devices with and without external light excitation. (d) Relationship of the photocurrent density with light intensity of the CuFeS2 nanocrystals and CuFeS2−PANi complex-based device. Inset shows the capacitance change as a function of light intensity. (e) Time-dependent current density change of CuFeS2 nanocrystals and CuFeS2−PANi-based devices with and without external magnetic field of 4000 Oe. (f) External magnetic-field-dependent MC of the CuFeS2 nanocrystals and CuFeS2‑PANi complex-based device. Inset shows the capacitance change as a function of external magnetic field.

heterojunction are shown in Figure 4d. The improved responsivity and response time of CuFeS2-polyaniline heterojunction could be attributed to the charge transfer interaction due to high electron affinity CuFeS2 and low ionization potential conjugated polyaniline. Furthermore, polyaniline is an effective hole transport material and facilitates the interconnection among CuFeS2 QDs, which significantly enhance the charge transfer and transport within the CuFeS2-polyaniline heterojunction. As shown in Figure 4d, the capacitance of CuFeS2-polyaniline heterojunction increases drastically with increasing external light illumination, indicating a strong charge transfer interaction between polyaniline and CuFeS2 QDs. Figure 4e shows the time dependent current density change under external magnetic field for CuFeS2 QDs and CuFeS2-polyaniline heterojunction. The magnetic response also becomes sensitive when the CuFeS2 nanocrystals are embedded in the polymer polyaniline matrix, in which the response time of CuFeS2-polyaniline heterojunction becomes much shorter (Figure 4e). The rise and decay time constants τ1 and τ2 of CuFeS2 QDs are 4.52 ± 0.21 s and 3.94 ± 0.17 s, respectively, whereas the response times τ1 and τ2 of CuFeS2-polyaniline heterojunction are 0.24 ± 0.05 s and 0.21 ± 0.07 s. The MC value is also increased by introducing the polyaniline matrix for the formation of hybrid heterojunction, which is shown in Figure 4f. Two possible mechanisms of enhanced MC effect are the following: One is due to the high charge transport rate of CuFeS2-polyaniline heterojunction

charge transport rate of magnetic semiconducting CuFeS2 QDs inhibits its practical significance. To address this, the conducting polymer polyaniline (EB) is introduced with CuFeS2 QDs to form the charge transfer heterojunction. Figure 4a shows the photoabsorption spectra of CuFeS2 QDs, polyaniline and the CuFeS2-polyaniline mixture. The CuFeS2 nanocrystals show a main peak around 500 nm while polyaniline polymer shows two absorption peaks at 326 and 615 nm, resulting from the excitation of benzene and quiniod segments.42 With the creation of CuFeS2-polyaniline heterojunction, it additionally shows a new and broad absorption tail at near-infrared, indicating the formation of new charge transfer band. As shown in Figure 4b, the current density of CuFeS2-polyaniline heterojunction drastically increases as compared to that of pure CuFeS2 QDs. Furthermore, CuFeS2-polyaniline heterojunction exhibits a much shorter response time than that of pure CuFeS2 QDs (Figure 4c). The rise and decay time constants could be obtained by fitting the temporal response with Ir(t) = Idark − C1exp[(t − t1)/τ1] and Id(t) = Idark + C2exp((t − t2)/τ2), where Idark is the dark current, C is the constant, τ1 and τ2 are the rise and decay time constants, respectively. The photoresponse times of CuFeS2 QDs, τ1 and τ2, are fitted as 18.6 ± 1.4s and 16.4 ± 1.7s, respectively, whereas the much faster response time τ1 and τ2 of CuFeS2-polyaniline heterojunction is 0.55 ± 0.12 s and 0.49 ± 0.13 s. The relationship between photocurrent density and light intensity of pure CuFeS2 QDs and CuFeS2-polyaniline 11218

DOI: 10.1021/acsami.6b03362 ACS Appl. Mater. Interfaces 2016, 8, 11215−11220

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compared to the pure CuFeS2 QDs. The other is external magnetic field induced intersystem crossing from the singlet to triplet charge-transfer state, leading to an increase of triplet exciton and polarons. The increased interaction between polarons and triplet excitons could increase the current density of CuFeS2-polyaniline heterojunction for the increased MC effect, as a result from the increased dissociation of triplet excitons to free charge carriers.43−45 In conclusion, organic−inorganic hybrid heterojunction, consisting of magnetic semiconducting CuFeS2 quantum dots and polyaniline polymer, exhibits a strong coupling between optical, electric, and magnetic degrees of freedom. The solution processing monodispersed CuFeS2 QDs, synthesized using a wet chemical hot injection method, exhibit a broad photoabsorption and magnetic characteristics which enable the coupling between optoelectronics and magnetoelectrics. As a result from the strong coupling between charge carriers and magnetic moments, the performance of CuFeS2 nanocrystals is strongly influenced by external electric and magnetic field. The fast and efficient response to external stimuli is further achieved in the hybrid system consisting of CuFeS2 and polyaniline. Our results open new avenues of solution-based low temperature processing for magnetic semiconducting organic−inorganic hybrid spin-electronic devices.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b03362. Experiment sections including materials synthesis, structure, spectral and device characteristic of CuFeS2 nanocrystals and CuFeS2−PANi complexes (PDF)

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AUTHOR INFORMATION

Corresponding Author

* E-mail: [email protected]. Tel.: (215) 204-2970. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Work at the Temple University (S.R.) was supported by the Army Research Office Young Investigator Program (W911NF15-1-0610, material design/self-assembly of carbon photovoltaics) and Department of Energy Basic Energy Sciences Award DE-SC0014902 (organic synthesis and physical property measurement).

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