Photoexcitation-Controllable Magnetization in Magnetic

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C: Physical Processes in Nanomaterials and Nanostructures

Photoexcitation-Controllable Magnetization in MagneticSemiconducting Nanohybrid Containing #-FeO-Graphene (0D-2D) Van Der Waals Heterostructure Based on Steady-State Pump-Probe Light Scattering Measurement in Magnetic Field 2

3

Hengxing Xu, Prem Prabhakaran, Sung Hyun Kim, Juhyung Jung, Rekha Narayan, Sang Ouk Kim, Kwang-Sup Lee, and Bin Hu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b11484 • Publication Date (Web): 05 Mar 2018 Downloaded from http://pubs.acs.org on March 5, 2018

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The Journal of Physical Chemistry

Photoexcitation-Controllable Magnetization in MagneticSemiconducting Nanohybrid Containing γ-Fe2O3Graphene (0D-2D) Van Der Waals Heterostructure Based on Steady-State Pump-Probe Light Scattering Measurement in Magnetic Field Hengxing Xu1, Prem Prabhakaran2, Sung Hyun Kim2, Juhyung Jung2, Rekha Narayan3, Sang Ouk Kim3, Kwang-Sup Lee2*, and Bin Hu1* Affiliations: 1

Department of Materials Science and Engineering, University of Tennessee, Knoxville, TN 37996-2100, USA

2

Department of Advanced Materials and Chemical Engineering, Hannam University, Daejeon 305-811, Republic of Korea

3

Department of Materials Science and Engineering, KAIST, Daejeon, 305-701, Republic of Korea

* Correspondence and requests for materials should be addressed to B.H. ([email protected]) and L. K-S. ([email protected])

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ABSTRACT This paper reports photoexcitation-enhanced magnetization in γFe2O3-Graphene (0D-2D) van der Waals heterostructure based on the experimental studies of steady-state pump-probe measurements in magnetic field. It is observed that applying a magnetic field on the γFe2O3/graphene material suspended in an organic solvent can cause a light scattering on the probe beam in the absence of pump beam, leading to a magnetic field effect of light scattering. This result shows that the magnetization on the γ-Fe2O3 nanoparticles can lead to a partial orientation of Fe2O3-graphene heterostructure components suspended in liquid. This effect of magnetic field on light scattering is caused by the dynamic magnetization of the superparamagnetic-semiconducting hybrid material. Interestingly, applying the pump beam functioning as photoexcitation can lead to an enhancement on the scattering of probe beam, increasing magnetic field effect of light scattering. The increased magnetic field effect of light scattering indicates that the photoexcitation from the pump beam applied to the hybrid enhances the magnetization on the γ-Fe2O3 nanoparticles through the d-π electron coupling between γFe2O3 and graphene in the hybrid material. This d-π electron coupling can be a practical method to develop photoexcitation-controllable magnetization through excited states based on chemically

linked

magnetic-semiconducting


hybrid

design.

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INTRODUCTION Photoexcitation-controllable magnetization offers unique principles of developing magnetooptical properties. Generally, semiconducting materials can exhibit significant response in charge transport and electric polarization under photoexcitation,1,2,3,4 while magnetic nanoparticles can be magnetized by applying external magnetic field due to the localized spins. 5,6 Incorporating magnetic nanoparticles into semiconducting materials in well-balanced systems provides a possible method to realize optical tuning on magnetic properties through the internal coupling between excited states in semiconductors and local spins in magnetic nanoparticles. In this work, we explore the possibilities of realizing the photoexcitation-controllable magnetization by chemically linking the magnetic γ-Fe2O3 nanoparticles and 2-D graphene based on the experimental studies of pump-probe measurements in magnetic field. The γ-Fe2O3 nanoparticles are well-known magnetic nanoparticles with size-dependent magnetization and coercivity.7,8,9 In particular, the γ-Fe2O3 nanoparticles allow the surface modification by using organic ligands,8 ready for chemically integrating with semiconducting materials towards the development of magneto-optical effects. On the other hand, the 2-D semiconducting graphene possesses highly anisotropic optic and magnetic properties 10,11,12,13,14,15,16,17,18,19 due to the coexisted delocalized and localized π electrons.10,11,12,13,14 Here, the semiconducting graphene is chemically linked with magnetic γ-Fe2O3 particles by using organic π-conjugated structures to form magneticsemiconducting hybrid materials. The pump-probe measurements in magnetic field are applied to the magnetic-semiconducting γ-Fe2O3/graphene hybrid materials suspended in organic solvent to explore photoexcitation-controllable magnetization. Specifically, the pump beam functions as a photoexcitation to generate excited states in the semiconducting graphene. When the suspended γ-Fe2O3/graphene hybrid material is placed in a magnetic field, the probe beam can be scattered



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by the semiconducting graphene upon magnetization-induced orientation. Therefore, the pumpprobe measurement leads to magnetic field effects of light-scattering in excited states to explore the

photoexcitation-controllable

magnetization

based

on

magnetic-semiconducting

γ-

Fe2O3/graphene hybrid materials. By using the pump-probe measurements in magnetic field, we find that the magnetization in the γ-Fe2O3 nanoparticles can be increased by applying a photoexcitation through excited states in semiconducting graphene in the γ-Fe2O3-graphene hybrid material. METHODS The single layer graphene was prepared by standard electrochemical exfoliation method using platinum as the counter electrode and a graphite flake as the working electrode. The γ-Fe2O3 nanoparticles were functionalized with the ligand Y and coupled with graphene in N-methyl pyrrolidone (NMP) solvent at room temperature. For light scattering measurement, the γ-Fe2O3 graphene nanohybrid were dispersed into NMP to form suspension with concentration of 2mg/ml. The suspension was sealed in a quartz vial placed in a magnetic field generated by an electrical magnet. The incident beam was provided from a YAG solid-state laser at 532 nm with an output intensity of 8 mW/cm2. The scattered light was collected by an optical fiber and measured by SPEX Fluorolog-3 spectrometer. For the MFE of light scattering measurement under excited state, pump beam from a He-Cd laser at 325 nm was used to excite the suspended nanohybrid. RESULTS AND DISCUSSION We have carried out experimental measurements on the transmission electron microscopy (TEM) images for γ-Fe2O3-graphene hybrid material (MYG), as shown in Figure 1a. The MYG nanohybrid was synthesized by coupling γ-Fe2O3 with electrochemically exfoliated graphene[20] with aid of a ligand Y. The ligand Y has a catechol residue with two hydroxyl groups at one end


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and a pyrene moiety at the other terminal, as given in the inset of Figure 1a. The catechol residue binds with the surface of the nanoparticle while the pyrene residue interacts with the surface of graphene through van der Waals interactions. The average size of the graphene flakes used to synthesize MYG is 1-3 microns. The nanoparticles were separately functionalized with the ligand Y and combined with graphene in N-methyl pyrrolidone (NMP) solvent at room temperature. The MYG nanohybrid is formed by the spontaneous self-assembly the functionalized γ-Fe2O3 onto graphene in NMP solution. Since the MYG nanohybrid of γ-Fe2O3 and graphene are assembled by van der Waals forces between the pyrene group in ligand Y and the surface of graphene, the MYG nanohybrid can be considered a zero dimensional-two dimensional (0D-2D) van der Waals heterostructure.21,22,23 The schematic of the setup for the magnetic field effect of light scattering (MFELS) measurement can be seen in Figure 1b, the suspension is sealed in a quartz vial placed in a magnetic field generated by an electrical magnet. A 532 nm continuous-wave laser is used as probe beam to generate scattered light in magnetic field, and a 325 nm continuous-wave laser is used as pump beam to generate excited states in both graphene and magnetic nanoparticles. The magnetic field is linearly increased for the MFELS measurement. It is kept constant at a given magnetic field strength during the time dependent measurement for analysis of magnetic torque generation under magnetic field. Absorption spectrum in Figure 1c reveals that MYG, graphene and the ligand Y have strong absorption below 400nm wavelength, which indicates that excited states can be generated within all these three components under 325nm laser excitation. The presence of characteristic pyrene peaks at 327 nm and 343 nm MYG is further evidence of the coupling of γ-Fe2O3 on to graphene. The ground state MFELS signal from MYG nanohybrid can be seen in Figure 2a. The amplitude of MFELS is defined as: MFE = (IB-I0)/I0, where IB and I0 are the scattered light



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intensities with and without external magnetic field. The light scattering intensity collected in the out of plane direction can be seen gradually increasing with applied magnetic field in Figure 2a. The scattering intensity gradually increases with the applied magnetic field and saturates at the MFELS amplitude of 12.4% for MYG. The enhancement in light scattering intensities is an indication that the MYG particles can be orientated parallel to the in-plane direction of magnetic field leading to larger reflection surface under magnetic field. The orientation of suspended particles can be determined by the magnetic energy generated by magnetic polarization, as shown in Eq. (1) 11 < >= ∙

∆

(1)

where θ is the angle between spin polarization and magnetizing field H, ∆ is the difference between the magnetic susceptibilities along different directions, V is the average volume of individual nanoparticles, C is the constant. The MFELS results are a clear evidence to show anisotropic magnetization in the suspended MYG particles upon applying a magnetic field. The time-dependent MFELS results measured under fixed magnetic field strength are given in Figure 2b. It can be seen that a higher MFELS amplitude can be generated under higher magnetic field. Under 450mT and 900mT magnetic field, MYG shows MFELS with amplitude of 12.1% and 15.3% respectively. More complete orientation of MYG flakes can be generated in presence of a stronger magnetic field leading to larger scattering cross-section, according to equation (1). This is the key to enhance MFELS amplitude. The MFELS of individual components of MYG are compared in Figure 2c. The MFELS of the ligands, the graphene flakes and γ-Fe2O3 nanoparticles were measured separately for this study. The ligand Y did not exhibit any notable MFELS while positive MFELS can be generated from electrolytically exfoilated graphene and γ-Fe2O3 magnetic nanoparticles (MNP). Pristine


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graphene suspension shows MFELS with lower amplitude (7%) than MFG suspension, while MNP shows the lowest amplitude around 3%. It has been shown that in graphene and its derivatives, defects such as vacancies, adatoms, and zigzag edges can introduce localized magnetic

moments

by

the

formation

of

unpaired

spins

leading

to

anisotropic

magnetism.12,13,14,15,16 We have previously reported that fluorinated graphene can be oriented by a magnetic field due to defect induced anisotropic magnetism, the MFELS results of electrolytically exfoilated graphene indicates that it may also incorporate such effects.17,24 It should be noted that MFELS of superparamagnetic γ-Fe2O3 MNP shows much narrower line-shape, revealing that MNP possesses a larger magnetic susceptibility giving rise to a faster magnetic response.25 By coupling MNP with graphene flakes through ligand Y, narrower MFELS line-shape and increase in MFELS amplitude can be observed in MYG suspension, indicating an enhancement of magnetic properties. The excited state effect on magnetization in MYG particles has been studied with MFELS measurement by adding a 325nm excitation beam. The MFELS results of MYG suspension in ground state and excited state are given in Figure 2d. An increase in MFELS amplitude can be observed in MYG suspension with photoexcitation. Under 250mW/cm2 photoexcitation, the MFELS amplitude increases from 12.2% to 14.8% in MYG suspension. In general, the 325nm pump beam can influence the MFELS amplitude through different channels including: interfering the detection of scattered light, generating photoluminescence, changing solvent viscosity through heating, and enhancing magnetic properties through excited states. In our steady state pump-probe measurement, the pump beam and probe beam are completely separated in wavelength by the spectrometer. Thus, the pump beam (325 nm) does not affect the scattering detection of probe beam (532 nm) at any incident angle. Furthermore, the MYG particles do not



have detectable photoluminescence under 325nm pump beam excitation, excluding the possibilities of having photoluminescence in scattering detection. In additional, the viscosity of solvent may be changed due to the heat generated by the pump beam, leading to a change in MFELS signal. However, based on our experimental analysis, the temperature change upon photoexcitation is less than 1K, which is not sufficient to introduce an appreciable change in MFELS signal via variation of solvent viscosity.17 Therefore, the enhanced MFELS amplitude can be induced by the enhancement of magnetic properties through excited states in graphene or MNP. According to the MFELS result from graphene and MNP, 325nm photoexcitation does not cause a notable change on MFELS signal in MNP suspension as seen in Figure 2e. In Figure 2f, MFELS signal in graphene suspension increases from 7.2% to 8.2% upon 250mW/cm2 photoexcitation. Therefore, the enhanced MFELS from both graphene and MYG under photoexcitation implies that MYG particles possess a stronger magnetization in excited state contributed by the excitation of the graphene part. It is known that delocalized π electrons can exhibit stronger electrical polarization in excited state based on molecular orbital theory. Due to the interaction between delocalized π electrons and localized spins in graphene flakes, it can be expected that a larger magnetization can be generated with enhanced electrical polarization in graphene under excited state. As a consequence, increased MFELS amplitude can be obtained from both graphene and MYG suspension upon photoexcitation due to the enhanced magnetization. To better understand the excited state effect on magnetization of MYG particles, we analyzed the magnetic response by comparing MFELS line-shape in ground state and under different intensity photoexcitations. Figure 3a and 3b are the MFELS results and normalized MFELS curves of MYG suspension under high intensity photoexcitation. Interestingly, increasing


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photoexcitation intensity leads to a narrower line-shape, in addition to the enhancement in MFELS amplitude in MYG suspension. However, no obvious line-shape narrowing phenomenon can be observed in the MFELS from graphene suspension under the influence of photoexcitation in Figure 2f. In general, a narrower line-shape implies that the orientation of suspended nanohybrid caused by magnetization occurs more easily due to magnetization under photoexcitation. Therefore, a line-shape narrowing phenomenon can provide an evidence that the magnetization becomes stronger under photoexcitation in the nanohybrid. Specifically, in the MYG nanohybrid, the graphene can interact with the γ-Fe2O3 nanoparticles through d-π electron coupling 26 , 27 based on (i) long-range Coulomb interactions and (ii) short-range spin-spin interactions between magnetic d electrons and semiconducting π electrons.28,29 When the MYG nanohybrid is placed in excited state upon photoexcitation, both Coulomb interaction and spinspin interaction between d electron in γ-Fe2O3 nanoparticles and delocalized π electron in graphene can be enhanced. Consequently, stronger magnetization can be generated in γ-Fe2O3 particles.

Therefore,

combining

the

superparamagnetic

γ-Fe2O3

nanoparticles

with

semiconducting graphene presents a convenient method to generate photoexcitation-controllable magnetization in the MYG nanohybrid. Time dependent MFELS results from MYG suspension at fixed magnetic field in ground and excited state are shown in Figure 3c and 3d. In time-dependent measurement, instead of gradually increasing the magnetic field, magnetic field is turned on and turned off immediately in order to analyze the magnetic response and relaxation of suspended particles in solvent The MYG particle under photoexcitation exhibit a shorter response time orienting under the influence of the magnetic field and reaching a saturated scattering intensity of light as seen in Figure 3d. Also the time for MYG particles to relax into random orientation is shorter in excited state. The



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magnetization of the MYG nanohybrid particles in solution by an applied magnetic field can lead to the gain of magnetic energy which can be defined as follows: 30 = − −

∆

(2)

Here µ is the magnetic moment, the orientation angle θ is the angle between magnetic moment and magnetic field. In the time dependent measurement, the suspended MYG nanoparticles are located at the states with different magnetic energy, labelled as U0 and UB, representing their energy without and with external magnetic field, respectively. After magnetic field is removed, the relaxation of suspended MYG nanoparticles is essentially driven by the restoring torque τ, given by =

∆

(3)

where ∆U = (UB – U0). In excited state, the total energy U consists of two components: magnetic energy U1 due to the magnetization of MYG nanoparticles in ground state, and the additional magnetic energy U2 under photoexcitation. The restoring torque τex in excited state then can be defined as

!

= " # $ + " $

(4)

When magnetic field is removed, the magnetic energy generated restoring torque can relax MYG particles back to random orientation. Clearly, a faster relaxation due to stronger restoring force suggests a larger magnetization of MYG particles in the excited state leading to faster relaxation process as shown in Figure 3d. To further understand the interaction between magnetic nanoparticles and the semiconducting graphene involved in photoexcitation-controllable magnetization, the effect of


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mixing MNPs into MYG and pristine graphene suspension liquids were examined. This is crucial in establishing the importance of assembling the van der Waals heterostructure of graphene and ligand functionalized γ-Fe2O3 MNPs. Both MYG and graphene suspensions were loaded with higher concentration of as synthesized MNPs and their MFELS were measured. The results from the MYG dispersions containing an added 0.1mg/ml of MNPs are given in Figure 4a. The result shows that MFELS signals can be further increased by the addition of MNPs. Furthermore, the line-shape of MFELS become narrower by adding MNP particles into MYG suspension as shown in the normalized MFELS in Figure 4b. The addition of MNPs to MYG can lead to complex interactions between the magnetization of free suspended superparamagnetic MNPs and the MNPs immobilized on graphene. The superior magnetic susceptibility of the MNPs lead to larger magnetization, this would mean that increasing the density of MNPs non-covalently assembled on graphene would lead to faster response in the MYG particle. The density of nanoparticles on the surface of the graphene are controlled by conditions of self-assembly the nanohybrid and should be optimized during synthesis of MYG. Interestingly when 0.1 mg/ml MNP is added into pristine graphene dispersion, the MFELS amplitude only increase slightly, while the line-shape become broader as shown in Figure 4c and Figure 4d. This means that the magnetization of the sample became gradual on addition of MNPs. In the absence of the ligand mediating the attachment of MNPs to the surface of graphene, the possible interaction between the MNPs and graphene would be through residual carboxylic or hydroxyl groups on the surface of graphene.31 Such an interaction is non-specific and random leading to sparse interaction between MNPs and graphene sheets and hence slower response to magnetism. In this scenario, it can be concluded that the MFELS amplitude increase shown in the case of MYG hybrid (Figure 4a) indicates additional interaction between the suspended MNPs and those assembled on graphene. Moreover,



the MFELS results presented in this study imply that the ligand (Y) mediated non-covalent assembly of MNPs on to graphene in the nanohybrid MYG leads to enhanced anisotropic magnetism.

CONCLUSIONS In summary, we experimentally study the magnetic properties of γ-Fe2O3/graphene hybrid materials by using magnetic field effect of light scattering. Our study shows that scattered light intensity from the γ-Fe2O3/graphene nanohybrid suspension can be enhanced upon applying magnetic field due to the orientation of MYG particles under external magnetic field. Specifically, we find that by chemically bonding graphene with MNP, the coupled sample can show much faster magnetic response as compared to pristine graphene. We observe that the amplitude of MFE of scattering from MYG suspension can be enhanced upon applying excitation beam, which indicates that photoexcitation can induce enhanced magnetization due to the interaction between delocalized π electrons and localized spins in graphene. Furthermore, loaded with higher concentration of as synthesized MNPs, MYG and graphene, show enhanced and deteriorated magnetic response, respectively. This result provides an evidence that mediated non-covalent assembly of MNPs on to graphene through ligands in the nanohybrid MYG can leads to enhanced anisotropic magnetism. Clearly, by using MFE of light scattering with pumpprobe method, our studies provide deeper understanding on the coupling between magnetic and semiconducting properties in nanohybrid with 0D-2D heterostructures.

AUTHOR INFORMATION


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Corresponding authors *Email: [email protected] *Email: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGEMENT This research was supported by the Asian Office of Aerospace Research and Development (AOARD, FA2386-12-1-4010 and FA 2386-15-1-4100).

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0.8 Absorption (a.u.)

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C

0.6

MNP Y Graphene MYG

0.4 0.2 0.0 300

350 400 450 Wavelength (nm)

500

Figure 1 a, TEM image to show morphology of magnetic nanoparticle functionalized graphene (MYG). The inset shows chemical structure of ligand Y. b, Schematic setup for measuring magnetic field effect of light scattering (MFELS) in excited state. c, UV-vis absorption spectrum of MYG, ligand (Y), as synthesized magnetic nanoparticles (MNP) and electrolytically exfoliated graphene.


Scattering change (%)

8 4 NMP(solvent)

12

c

9

MYG

Graphene

6 MNP

3 0

Ligands (Y)

0

4

e

300 600 900 Magnetic field (mT) 2

250mW/cm

3 2

0mW/cm

2 1 0

MNP exc @325nm

0

16

300 600 900 Magnetic field (mT)

B

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b

B

off

on

12 8

900mT

4

450mT

0

MYG

0

300 600 900 Magnetic field (mT) Scattering change (%)

0 0


MYG


12 a


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



60 120 Time (s)

16 d

2

Excitation 250mW/cm

12 8 4

2

Excitation 0mW/cm

0

MYG exc @ 325nm

0


9 f

2

250mW/cm

6 2

0mW/cm

3 0

Graphene exc @325nm

0


Figure 2 a, MFELS signals from magnetic nanoparticle functionalized graphene suspended in NMP with the concentration of 1mg/ml. b, Time-dependent MFELS profile under different magnetic field c, MFELS signals from coupled sample (MFG) and separated components (MNP, graphene, organic molecules). d, MFELS signals from magnetic nanoparticle functionalized graphene in ground state and excited state. MFELS signals from e, magnetic nanoparticles and f, graphene.


16

2

Excitation 500mW/cm

12 8

2

Excitation 250mW/cm

4

2

Excitation 0mW/cm

0

MYG exc @ 325nm

0


Bon

c

Boff

2

250mW/cm

12 8 4

2

0mW/cm

0

MYG exc @ 325nm

0

50

100 Time (s)

150

Normalized change (%)

16 a

1.0 b 0.8

2

Excitation 500mW/cm

0.6

Excitation 250mW/cm

0.4 0.2

Excitation 0mW/cm

0.0

MYG exc @ 325nm



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



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0

2

300 600 900 Magnetic field (mT) Bon

1.0

2

Boff

0.8

d 2

250mW/cm

0.6 0.4 2

0mW/cm

0.2 0.0

MYG exc @ 325nm

0

50

100 Time (s)

150

Figure 3 a, MFELS signals from MYG in ground and excited states b, normalized MFELS curve of MYG in ground state and excited state. Time-dependent profile of MFELS signals from MYG under c. ground state and d. excited state.


0mg/ml MNP

MYG with MNP


6 c 4 0.1mg/mL MNP

2 0mg/mL MNP

0 Graphene with MNP

0


1.2 b 1.0 0.8 0.6 0.4 0.2 0.0 -0.2 0


0.1mg/ml MNP


15 a 12 9 6 3 0 -3 0


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0.1mg/ml MNP

0mg/ml MNP

MYG with MNP


1.0 d 0.8 0.6

0.1mg/mL MNP

0.4 0mg/mL MNP

0.2 0.0

Graphene with MNP

0


Figure 4 a, MFELS signals from MFG with MNP added. b, normalized MFELS curves of MFG with MNP added. c, MFELS signals from graphene with MNP added. d, normalized MFELS curves of graphene with MNP added.


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TOC Graphic


Photoexcitation-Controllable Magnetization in Magnetic

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