Resolving the Structure and Magnetic Properties of FePc/CdSe Organic/Inorganic Semiconducting Nanocomposite

doi:10.21203/rs.3.rs-4886169/v1

The exploration of diverse methodologies for creating hybrid organic/inorganic semiconducting nanocomposites represents a significant area of scientific inquiry, driven by the escalating demand to cater to magnetic-based technologies. In this work, we employed a straightforward solid-state calcination process to fabricate FePc/CdSe QDs nanocomposites with two distinct ratios, specifically 1:1 and 3:5. Structural analysis and the assessment of magnetic properties of these nanocomposites were conducted utilizing a comprehensive array of techniques, including X-ray diffraction (XRD), high-resolution transmission electron microscopy (HR-TEM), Mössbauer spectroscopy, and vibrating sample magnetometry (VSM). Our investigation substantiates the formation of nanocomposites comprising FePc molecules, CdSe QDs, and γ-Fe₂O₃ nanoparticles. Notably, these nanocomposites demonstrate a soft ferromagnetic behaviour, serving as a stark contrast to the diamagnetic behaviour observed in original CdSe QDs.

Physical sciences/Nanoscience and technology/Nanoscale materials/Magnetic properties and materials

Physical sciences/Nanoscience and technology/Nanoscale materials/Organic inorganic nanostructures

Physical sciences/Nanoscience and technology/Nanoscale materials/Quantum dots

Physical sciences/Nanoscience and technology/Nanoscale materials/Structural properties

Physical sciences/Nanoscience and technology/Nanoscale materials/Synthesis and processing

The exchange effect between magnetic ions and charge carriers in diluted magnetic quantum dots (DMQDs) has emerged as a focal point of research [1, 2]. The significant potential of DMQDs is evident in the initial development of spintronics devices, such as magnetic inductors, spin field-effect transistors (spin-FETs), and high-density non-volatile memory [3]. More recently, DMQDs have played a key role in developing quantum information devices [4]. This novel category of nanomaterials is poised to yield enhanced efficiency and miniaturization owing to their quantum confinement effect, thereby surpassing bulk diluted magnetic semiconductors (DMS). Consequently, numerous methodologies have been undertaken to incorporate transition metal elements into various quantum dots (QDs). These methods include the standard direct doping [5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 23, 17, 18, 19, 20, 21, 22, 23, 24] as well as the less common core-shell [25, 26, 27, 28, 29, 30] method. Notably, significant attention has been directed towards II-VI QDs, as these transition metals have exhibited higher solubility in the II-VI QD host matrices [31, 32].

In the realm of semiconductor research, the combination of quantum dots (QDs) with organic semiconductors has recently emerged as an alluring strategy for the modification and enhancement of their intrinsic structural [33, 34], electronic [35], optical [36, 37, 38, 39, 40, 41], magnetic [42, 43], and optoelectronic [44, 45] properties.

In particular, a recent development involves the creation of CdS/iron phthalocyanine (FePc) core/shell QDs through a calcination technique, with subsequent evaluation of their viability for employment in photocatalytic applications [39] without investigating their potential magnetic properties. The solid-state reaction method offers the advantage of facilitating close contact between the incorporated organic molecule and the host QDs.

Within this context, we introduce a thorough investigation of the structural and magnetic properties of the FePc/CdSe QDs nanocomposites synthesized using a solid-state reaction method. Our investigation involves using X-ray diffraction (XRD), Mössbauer spectroscopy, and vibrating sample magnetometer (VSM) techniques to gain a comprehensive understanding of their characteristics.

Preparation of Nanocomposites

Nanocomposites (NCs) are synthesized using the solid-phase reaction method [39]. The procedure involves grinding a specific ratio of CdSe quantum dots (QDs), previously synthesized as described [40], and iron Phthalocyanine (FePc, Acros organic, 96%), together in a mortar for 30 minutes with the addition of 1 mL of ethanol until a dark green powder is achieved. Subsequently, the resulting powder is placed in a covered porcelain crucible and heated in a muffle furnace at 300°C for 1 hour, yielding a reddish-brown powder. Two NC ratios have been obtained using this method: 1:1 and 3:5 of FePc:CdSe QDs ratios. These ratios, before (BC) and after (AC) the calcination process, are designated as 1:1 NC-BC, 1:1 NC-AC, 3:5 NC-BC, and 3:5 NC-AC, respectively.

Characterization Techniques

The optical absorption spectra of all samples were conducted using the Jasco model (V-570) spectrophotometer in the 400-800 nm wavelength range. The X-ray diffraction (XRD) charts were measured at room temperature using a Panalytical (X’Pert Pro MRD) X-ray diffractometer with CuKα- radiation operated at a voltage of 45 kV and a current of 30 mA in the angular range from 4° to 80° (2θ). High-resolution transmission electron microscope (HR-TEM) images and electron diffraction (ED) patterns were acquired using a JEOL® JEM-2100LaB6 apparatus operating at 120 kV and equipped with a 4 Megapixel Gatan® CCD camera (Orius SC200B). 57Fe Mössbauer spectroscopy measurements were performed at room temperature in transmission geometry on a WissEl spectrometer operating in constant acceleration mode. A 57Co/Rh γ-radiation source with a maximum activity of 50 mCi was used. The velocity scale was calibrated using a α-Fe absorber. Finally, the vibrating sample magnetometer (VSM) equipment (Lakeshore 7410) was used to investigate the magnetic behavior of all samples at room temperature.

The optical absorption spectra for all prepared samples, both before and after calcination, were recorded and are depicted in Figure 1. The pure FePc spectrum exhibits two primary absorption peaks at 655 nm and 695 nm, corresponding to energy levels of 1.89 eV and 1.78 eV, respectively, attributed to the FePc Q-band transitions [46]. These peaks persist in the non-calcinated nanocomposite samples, indicating an absence of interaction between the FePc molecules and CdSe QDs at this stage.

Subsequently, following the calcination process, the absorption peaks in the nanocomposite spectra reveal two broad absorption transitions at approximately 587 nm and 630 nm, corresponding to energy levels of 2.10 eV and 1.97 eV, respectively. The observed alterations in the peaks’ positions may be ascribed to a rigid blue shift of approximately 0.21 eV in the original FePc Q-band transitions, stemming from the interaction with the CdSe QDs.

Figure 1: The UV-visible optical absorption spectra of (bottom to up); the pure CdSe quantum dots, as well as the 3:5 and 1:1 nanocomposites (NC) before (BC) and after (AC) the calcination process and FePc molecules. The blue lines were employed to delineate the precise position of the FePc Q-band transitions. Additionally, black lines were utilized to denote the emergence of new phases and/or the Q-band shift of two peaks in the post-calcination samples. Furthermore, green lines were employed to represent the relative intensities between the original FePc Q-band transitions and the two new peaks observed subsequent to the calcination process.

It is necessary to observe that the intensity of both peaks is nearly identical, contrary to their original presentation in the pure FePc spectrum or the nanocomposites' spectra prior to the calcination process, see the difference of the green line slope of each spectrum in Figure 1. This discrepancy in the relative intensities of the peaks may plausibly be ascribed to the emergence of a new phase within the nanocomposite. Notably, both scenarios, i.e. the shift in the FePc Q-band transitions and the emergence of a new phase, are not mutually exclusive and can coexist concurrently.

Utilizing the transmission electron microscope, we conducted an analysis to ascertain the evolution of the nanoparticle sizes subsequent to the calcination process for both nanocomposites compared to the pure CdSe QDs, as illustrated in Figure 2. The examination revealed that the particle size distribution of the 3:5 NC-AC nanocomposite ranges from 10 to 25 nm, with the average size being 17 nm, as indicated in Figure 2 (b). The 1:1 NC-AC nanocomposite, conversely, demonstrates a particle size distribution ranging from 22 to 30 nm, with an average of 25 nm, as depicted in Figure 2 (c).

Retrieving the previously recorded 4 nm particle size for the pure CdSe QDs [41], see Figure 2(a), it becomes apparent that the nanocomposite particle size has significantly changed following the introduction of FePc molecules. This can be attributed to the probability that the presence of FePc molecules results in the shell formation around the CdSe QDs [39, 43]. In other words, adding more FePc molecules leads to an increase in the particle size.

Additionally, we recorded the electron diffraction (ED) patterns for both nanocomposites. In the electron diffraction pattern of the 3:5 NC-AC sample, as seen in the inset of Figure 2 (a), distinct diffraction rings corresponding to the CdSe wurtzite structure are evident. It is important to note, however, that the pattern is observed to be blurred. In the ED pattern of the 1:1 NC-AC sample, see the inset of Figure 2 (b), the discernible presence of CdSe wurtzite diffraction rings is evident. The diffraction pattern reveals the presence of additional phases, evident from the diffraction dots dispersed throughout the pattern background. This indicates the existence of a new phase, as alluded to in the preceding section. Unfortunately, due to camera tilt, it is challenging to derive precise data from the illustrated figures.

Figure 2: The high-resolution transmission electron images (HR-TEM) for; (a) the pure CdSe QDs, (b) the 1:1 NC-AC, and (c) the 3:5 NC-AC samples.

Recording the X-ray diffraction (XRD) patterns for all pure and nanocomposite samples, shown in Figure 3, gives us insight into the structural evolution of our nanocomposites. In detail, the XRD pattern depicted in Figure 3 (a) displays distinct diffraction peaks at 23.3°, 25.4°, 41.8°, 45.0°, and a shoulder at 27.1°. These peaks can be attributed to the CdSe wurtzite structure planes: (100), (002), (110), (103), and (101), respectively [47, 48]. The peaks observed at angles below 20° are ascribed to the organic surfactants employed in the synthesis process. In addition, the presence of peaks at 31.1° indicates the (101) plane of the hexagonal pure phase of Se [49]. In the upper panel of Figure 2, i.e., Figure 3 (d), the distinct diffraction peaks observed for pure FePc molecules are as follows: 7.3°, 9.2°, 13.9°, 15.5°, 18.5°, 23.5°, and 28.9°. These peaks correspond to the characteristic crystallographic planes , , , , , , and of the monoclinic structure of FePc with a β-phase [50, 51].

The XRD patterns of the non-calcinated nanocomposites, i.e. 1:1 NC-BC and 3:5 NC-BC, shown in Figure 3 (b), display the coexistence of the FePc and CdSe diffraction peaks, confirming that the materials are not yet interacting as suggested by the optical absorption spectra. Following the calcination process, Figure 3(c). residual traces of the original pure phases are discernible in both nanocomposites. Furthermore, distinct diffraction peaks have emerged, unrelated to the pure phases. The identified peaks at 20.2°, 31.1°, 36.5°, and 44.3° are attributed to the (111), (220), (311), and (400) planes of the cubic spinel structure of maghemite γ-Fe₂O₃ structure [52, 53] and or the Fe₃O₄ magnetite structure [52, 54]. Notably, the 1:1 NC-AC sample clearly exhibits more prominent XRD diffraction peaks of FePc and iron oxide phases compared to the 3:5 NC-AC sample. This may be attributed to the initially higher concentration of FePc molecules used to synthesize the 1:1 NC-AC nanocomposite.

Figure 3: The X-ray diffraction charts for; (a) pure CdSe QDs, (b) the 1:1 and 3:5 NCs before calcination (BC), (c) 1:1 and 3:5 NCs after (AC) the calcination, and (d) FePc molecules.

For the investigation of the Fe ions phases in all systems, M?ssbauer spectroscopy was performed on the pure FePc sample and the calcinated nanocomposites under ambient conditions, as depicted in Figure 4. The spectrum represented in Figure 4(a) closely resembles the spectrum of the pure FePc molecules studied previously [43]. Based on the analysis of the prior study [43], it is evident that the β-FePc exhibits three distinct doublet phases, Table 1. Phases 1 and 2 (Ph1 & Ph2) correspond to the high-spin and low-spin states of oxygenated β-FePc adducts (HS – OX & LS – OX), respectively. Meanwhile, the third phase (Ph3) aligns with the high spin configuration of the pristine state of the β-FePc (HS – β-FePc). Consequently, the combination of all phases corresponds to a superparamagnetic material, as discussed earlier [43].

The M?ssbauer spectrum of the 1:1 NC-AC sample presents the existence of six distinct phases, three doublets and three sextets, as depicted in Figure 4 (b). The first two doublets present in the sample are the two oxygenated phases of the β-FePc adducts (Ph1 & Ph2), accounting for about 6% of the Fe ions. Another doublet has been resolved, Ph4, that closely resembles the superparamagnetic (SPM) phase of γ-Fe₂O₃ nanoparticles [55]. This phase encompasses approximately one-third of the Fe ions within the 1:1 NC-AC sample, as estimated from its area ratio (AR) indicated in Table 1. Based on their isomer shift (IS) and quadrupole splitting (QS) parameters indicated in Table 1, the Ph5 sextet is attributed to the tetrahedral (A-site) Fe3+ ions present in magnetite γ-Fe₂O₃ nanoparticles [55], while the Ph6 sextet aligns with the octahedral (B-site) Fe3+ ions in γ-Fe₂O₃ [55]. It should be noted that both phases possess a high spin configuration of ferromagnetic (F) behaviour and, therefore, a more magnetic effect should be observed, as will be revealed later. The collective phases account for roughly 24% of the iron ions present in the sample as determined by their AR. The third sextet, Ph7, resembles a unique state of intermediate state between the SPM and the F phase of the Fe3+ ions in γ-Fe₂O₃by expressing a weaker hyperfine field (HF) parameter [56] compared to those of Ph5 and Ph6 as can be noted in Table 1. This phase resembles about 34%, i.e. one-third, of the whole Fe3+ ions present in the sample. Notably, the observed hyperfine magnetic field values are lower than the standard values for iron oxide. This variance can be ascribed to the reduction in the particle size of our composite [57]. This observation aligns with our prior analysis of the TEM image.

In the Mössbauer spectrum of the 3:5 NC-AC sample, Figure 4(c), four primary doublets are discernible. The initial three doublets resemble the Ph1, Ph2, and Ph3, mirroring those of the FePc discussed previously. The three distinct phases collectively represent approximately 16.5% of the iron content within this composite. The fourth doublet corresponds to the Ph4 phase, i.e. of the SPM phase of γ-Fe₂O₃ nanoparticles, albeit with slightly different IS values and notably distinct QS parameters. These distinctions are ascribed to the divergent local environments experienced by Fe ions in the nanocomposite sample compared to the 1:1 NC-AC one. Considering the evident absence of Ph5, Ph6, and Ph7 in the 3:5 NC-AC sample, it is anticipated that the magnetic behaviour of the 1:1 NC-AC sample will surpass that of the 3:5 NC-AC or pure samples. Moreover, this confirms that the average particle size in the 1:1 NC-AC sample exceeds that of the 3:5 NC-AC sample, as deduced from the TEM images.

Figure 4: The Mössbauer spectra for; (a) pure FePc molecules, (b) 1:1 NC-AC, and (c) 3:5 NC-AC samples. The different peak fillings resemble the different iron phases; refer to the text.

It should be noted that the analysis of the Mössbauer spectra of both 1:1 and 3:5 NC-AC samples clearly shows that the identified oxide phase in both samples corresponds only to the γ-Fe₂O₃ nanoparticles, excluding the presence of any Fe₃O₄ traces, as was suggested earlier in the XRD analysis. Notably, the γ-Fe₂O₃ nanoparticles are known to have an optical band gap of about 2.2 eV [58], corresponding to a wavelength of about 565 nm. Therefore, the concurrent presence of FePc molecules and γ-Fe₂O₃ nanoparticles in the post-calcinated samples provides evidence supporting the observation of two equally intense peaks at approximately 2.10 eV (587 nm) and 1.97 eV (630 nm) in the optical absorption spectra delineated by the green lines in Figure 1. This phenomenon results in a blue shift of roughly 0.21 eV from the original FePc Q-band transitions, likely attributed to a plausible interaction between the FePc molecules and the CdSe QDs surface.

Figure 5 depicts the vibrating sample magnetometry (VSM) measurements for all nanocomposite samples in conjunction with the pure CdSe QDs and FePc molecules. As detected in previous studies, the pure FePc molecules exhibit superparamagnetic properties at room temperature with a saturation magnetization (Ms) of about 390 memu/g [43]. In contrast, CdSe QDs exhibit reliable diamagnetic behaviour attributed to the absence of any unpaired electrons in their outer energy shells [59]. Adding FePc molecules to the CdSe QDs in a nanocomposite form transforms the diamagnetism into a magnetic state, as depicted in Figure 5. In general, all the nanocomposite specimens display a transition from superparamagnetic characteristics pre-calcination to a significantly soft-ferromagnetic behaviour post-calcination.

Specifically, the 3:5 NC-BC sample manifests an initial Ms value of approximately 100 memu/g, Table 2. Subsequent to calcination, i.e. the 3:5 NC-AC sample, the saturation value experiences a threefold increase, reaching an approximate magnitude of 315 memu/g, as illustrated in Figure 5(a) and tabulated in Table 2. Furthermore, the 3:5 NC-AC sample presents a coercive field (Hc) of around 5 G and a remnant magnetization (Mr) of 0.3 memu/g at room temperature indicative of very soft-ferromagnetic behaviour, as evidenced in the inset of Figure 5(a). Notably, the significant change in the Ms values before and after the calcination process is likely attributable to the interaction with the CdSe QDs and/or the formation of γ-Fe₂O₃ nanoparticles, as concluded by the Mössbauer analysis above.

In the 1:1 NC system, Figure 5 (b), the NC-BC nanocomposite sample shows a Ms value of about 250 memu/g, i.e. higher than its 3:5 counterpart. This can be ascribed to the higher proportion of FePc molecules utilized in the synthesis process of the 1:1 system than in the 3:5 one. Upon calcination, i.e., 1:1 NC-AC, the Ms attains a value of approximately 420 emu/g, a Hc of about 12 G, and a Mr of 1 memu/g, Table 2. The Hc and Mr of the 1:1 NC-AC exhibit higher values compared to those of the 3:5 NC-AC counterpart, indicating a marginally stronger soft-ferromagnetic behaviour. Interestingly, the Ms value of the 1:1 NC-AC sample exceeds even that observed for the pure FePc phase. This is most likely attributed to the γ-Fe₂O₃ phase that were confirmed above by the XRD and Mössbauer data analysis.

Considering all the aforementioned factors, it appears that integrating FePc molecules into the CdSe QDs matrix in a nanocomposite structure has the potential to convert their originally diamagnetic behaviour into a soft-ferromagnetic one. This transformation renders them suitable for deployment in a wide range of spin-based devices that rely on semiconducting host materials exhibiting diluted spin-polarization [60].

Figure 5: The vibrating sample magnetometer (VSM) spectra for; (a) 3:5 NC and (b) 1:1 NC before and after calcination compared with the spectrum of the pure FePc molecules.

In conclusion, we have successfully synthesized FePc/CdSe nanocomposites in two distinct ratios, 1:1 and 3:5, employing the solid-state calcination method. The interaction with the CdSe QDs led to a blue shift of approximately 0.2 eV in the Q-band transition peaks of the FePc molecules. Moreover, the calcination method facilitated the formation of iron oxide phases alongside the pure phases of both FePc molecules and CdSe QDs within the nanocomposite powder, as evidenced by XRD analysis. Notably, Mössbauer analysis verified that the iron oxide phase in both nanocomposites is attributed to γ-Fe₂O₃ nanoparticles, suggesting a stronger magnetic behaviour in the 1:1 NC sample compared to the 3:5 NC. This finding was further corroborated by the VSM charts, indicating a transition in magnetic behaviour from entirely diamagnetic in the pure CdSe QDs to firstly superparamagnetic, and finally to soft ferromagnetic in both nanocomposites, before and after-calcination process, respectively. Consequently, this methodology presents an alternative approach to impart a magnetic effect to the diamagnetic inorganic CdSe QDs by employing magnetic organic molecules such as FePc.

Data Availability

The datasets used and/or analysed during the current study available from the corresponding author on reasonable request.

Author Contributions Statement

E.G., Sh.A.S., and M.A.M. participated in performing the measurements.

T.Z.A., H.H.E., and A.E. participated in analyzing all the data and supervised the work

A.E. conceptualized the idea, prepared the figures, and wrote the main manuscript.

All authors reviewed the manuscript.

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Table 1: The parameters for analyzing Mössbauer spectra include isomer shift (IS), quadrupole splitting (QS), hyperfine field, area ratio (AR), and phase identifications.

Sample	Phase	IS (mm/s)	QS (mm/s)	HF (T)	AR (%)	Identification
FePc	Ph1	0.373	2.566	---	48.5	Fe³⁺: HS – OX of β-FePc
	Ph2	0.270	0.856	---	45.9	Fe³⁺: LS – OX of β-FePc
	Ph3	0.616	1.077	---	5.6	HS – β-FePc
1:1 NC-AC	Ph1	0.379	2.428	---	3.5	Fe³⁺: HS – OX of β-FePc
	Ph2	0.274	0.872	---	2.2	Fe³⁺: LS – OX of β-FePc
	Ph4	0.375	0.930	---	33.4	Fe³⁺: Superparamagnetic phase NP
	Ph5	0.288	0.077	47.8	12.6	Fe³⁺: A-site (γ-Fe₂O₃)
	Ph6	0.374	-0.094	51.1	11.3	Fe³⁺: B-site (γ-Fe₂O₃)
	Ph7	0.268	0.040	41.3	34.3	Fe³⁺: Intermediate phase in γ-Fe₂O₃
3:5 NC-AC	Ph1	0.301	2.528	---	5.8	Fe³⁺: HS – OX of β-FePc
	Ph2	0.268	0.869	---	8.0	Fe³⁺: LS – OX of β-FePc
	Ph3	0.624	1.103	---	2.5	HS – β-FePc
	Ph4	0.397	0.907	---	83.7	Fe³⁺: Superparamagnetic phase NP

Table 2: The magnetic parameters estimated from Figure 5: remnant magnetization (Mr), saturation magnetization (Ms), coercive field (Hc), and their corresponding magnetic effect.

Sample	Mr	Ms	Hc (G)	Identification
Sample	(memu/g)		Hc (G)	Identification
3:5 NC-BC	---	100	---	Superparamagnetic
3:5 NC-AC	0.3	315	5	Soft-Ferromagnetic
1:1 NC-BC	---	250	---	Superparamagnetic
1:1 NC-AC	1	435	12	Soft-Ferromagnetic
FePc	---	390	---	Superparamagnetic

No competing interests reported.

Resolving the Structure and Magnetic Properties of FePc/CdSe Organic/Inorganic Semiconducting Nanocomposite

Status:

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Abstract

Figures

Introduction

Experimental work

Results and Discussion

Conclusion

Declarations

Author Contributions Statement

References

Tables

Additional Declarations

Status:

Version 1