Nanotechnology

ACCEPTED MANUSCRIPT

Reduced graphene oxide-germanium quantum dot nanocomposite: Electronic, optical and magnetic properties To cite this article before publication: Tabitha Amollo et al 2017 Nanotechnology in press https://doi.org/10.1088/1361-6528/aa9299

Manuscript version: Accepted Manuscript Accepted Manuscript is “the version of the article accepted for publication including all changes made as a result of the peer review process, and which may also include the addition to the article by IOP Publishing of a header, an article ID, a cover sheet and/or an ‘Accepted Manuscript’ watermark, but excluding any other editing, typesetting or other changes made by IOP Publishing and/or its licensors” This Accepted Manuscript is © 2017 IOP Publishing Ltd.

During the embargo period (the 12 month period from the publication of the Version of Record of this article), the Accepted Manuscript is fully protected by copyright and cannot be reused or reposted elsewhere. As the Version of Record of this article is going to be / has been published on a subscription basis, this Accepted Manuscript is available for reuse under a CC BY-NC-ND 3.0 licence after the 12 month embargo period. After the embargo period, everyone is permitted to use copy and redistribute this article for non-commercial purposes only, provided that they adhere to all the terms of the licence https://creativecommons.org/licences/by-nc-nd/3.0 Although reasonable endeavours have been taken to obtain all necessary permissions from third parties to include their copyrighted content within this article, their full citation and copyright line may not be present in this Accepted Manuscript version. Before using any content from this article, please refer to the Version of Record on IOPscience once published for full citation and copyright details, as permissions will likely be required. All third party content is fully copyright protected, unless specifically stated otherwise in the figure caption in the Version of Record. View the article online for updates and enhancements.

This content was downloaded from IP address 155.69.24.171 on 17/10/2017 at 14:17

Page 1 of 38

pt

Reduced graphene oxide-germanium quantum dot nanocomposite: Electronic, optical and

us cri

magnetic properties Tabitha A. Amollo,1 Genene T. Mola,2 and Vincent O. Nyamori1* 1

University of KwaZulu-Natal, Westville Campus, School of Chemistry and Physics,

Private Bag X54001, Durban 4000, South Africa. 2

University of Kwazulu-Natal, Pietermaritzburg Campus, School of Chemistry and Physics,

an

Private Bag X01, Scottsville, 3209, South Africa.

dM

Corresponding author e-mail address: [email protected]

Abstract

Graphene provides numerous possibilities for structural modification and functionalization of its carbon backbone. Localized magnetic moments can, as well, be induced in graphene by the formation of structural defects which include vacancies, edges, and adatoms. In this work,

pte

graphene was functionalized using germanium atoms, we report the effect of the Ge ad atoms on the structural, electrical, optical and magnetic properties of graphene. Reduced graphene oxide (rGO)-germanium quantum dot nanocomposites of high crystalline quality were synthesized by

ce

the microwave-assisted solvothermal reaction. Highly crystalline spherical shaped germanium quantum dots, of diameter ranging between 1.6-9.0 nm, are anchored on the basal planes of rGO. The nanocomposites exhibit high electrical conductivity with a sheet resistance of up to 16 Ω sq1

. The electrical conductivity is observed to increase with the increase in Ge content in the

Ac

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

AUTHOR SUBMITTED MANUSCRIPT - NANO-115161.R1

nanocomposites. High defect-induced magnetization is attained in the composites via germanium

1

AUTHOR SUBMITTED MANUSCRIPT - NANO-115161.R1

pt

adatoms. The evolution of the magnetic moments in the nanocomposites and the coercivity showed marked dependence on the Ge quantum dots size and concentration.

Quantum

us cri

confinement effects is evidenced in the UV-Vis absorbance spectra and photoluminescence

emission spectra of the nanocomposites which show marked size-dependence. The composites manifest strong absorption in the UV region, strong luminescence in the near UV region, and a moderate luminescence in the visible region. Keywords

magnetization, coercivity 1. Introduction

an

Reduced graphene oxide, germanium quantum dots, electrical conductivity, quantum confinement,

dM

Graphene has attracted huge research interest in a vast research landscape of diverse applications such as condensed matter physics [1], electronics [2], optoelectronics [3,4], energy conversion [5] and energy storage [6,7]. It consists of a single layer of sp2-bonded carbon atoms packed in a dense honeycomb lattice structure. The sp2 hybridized graphene has in-plane σ-bonds and out-ofplane pi-bonds normal to the plane [8]. Its partially filled π-orbitals give rise to a network of

pte

delocalized electrons which is responsible for the electronic transport in graphene [9]. Pristine graphene is a semimetal or zero energy band gap semiconductor with a completely-filled valence band and an empty conduction band crossing linearly at the Fermi level (EF) [10]. It exhibits high

ce

charge carrier mobility and electrical conductivity [11]. Charge transport in graphene is ballistic as its zero effective mass charge carriers can travel within sub-micrometer distances of its lattice without getting scattered [12]. Hence, it is a promising material for nanoelectronics devices.

Ac

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

Page 2 of 38

However, practical application of pristine graphene in electronics is limited by its zero-gap energy band structure. Similarly, graphene is characterized by weak spin-orbit coupling and long spin 2

Page 3 of 38

pt

relaxation lengths resulting from hyperfine coupling; these properties make it an ideal candidate for spintronic applications like spin-based quantum information processing and spin-current

us cri

polarized transport [13]. Graphene’s application in spintronics is however, limited by its lack of

intrinsic magnetic moments and the long spin relaxation length [14]. Nonetheless, graphene afford various possibilities for structural modification and functionalization [15]. Doping is one suitable approach of tailoring the structural and electronic properties of graphene [16,17]. Localized magnetic moments can be induced in graphene via defects like vacancies, edges, and adatoms, as well as localization of electrons around the defects which can result in spin current modulation

an

[18,19]. Functionalization of graphene has been done mostly by nitrogen and boron atoms since these atoms have same sizes as carbon atoms. Likewise, graphene is portentous for developing

dM

nanocomposites with desired mechanical, thermal, electronic, optical and magnetic properties. Germanium nanoparticles exhibit small tunable energy band gap (0.67 eV) [20], large exciton Bohr radius which allows for observable quantum confinement effects [21], high carrier mobility [20] and a high absorption coefficient of approximately 200,000 cm-1 at 2 eV [22]. Thus, it has become a material of interest for application in optoelectronics [23], energy conversion (solar

pte

cells) [24] and energy storage (Li-ion batteries) [25]. Experimental and theoretical studies have shown that Ge can form 2-D stable single layer honeycomb structure, just like graphene [26-28]. The 2-D Ge exhibits a band structure like that of graphene. It has bands that cross linearly at the

ce

EF, charge carriers behaving as massless Dirac Fermions around the EF, and its bands exhibit the ambipolar characteristics [26,29]. On the other hand, germanium quantum dots exhibit quantum confinement effects which are observable in optical experiments [30,31]. Generally, quantum confinement effects increase the oscillator strength of semiconductor nanostructures and hence,

Ac

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

AUTHOR SUBMITTED MANUSCRIPT - NANO-115161.R1

makes them more viable for application in optoelectronics.

3

AUTHOR SUBMITTED MANUSCRIPT - NANO-115161.R1

pt

In this work, graphene was functionalized using Ge ad atoms. Since both graphene and Ge can form stable 2-D honeycomb lattice structure as aforementioned, this study sought to elucidate

us cri

the effect of Ge ad atoms on the magnetic, electronic and optical properties of graphene. Reduced graphene oxide-germanium quantum dot (rGO-Ge) nanocomposites were synthesized and

characterized for structural, magnetic, electronic and optical properties. Reduced graphene oxidegermanium/germanium dioxide nanocomposites have been investigated for application in Li-ion batteries as anode materials [6,25,32]. In such application, Ge is functionalized by rGO which serves to improve the electrochemical performance and cycling stability of Ge as an anode

an

material. We examined the functionalization of rGO by Ge ad atoms and explored the potency of the modified rGO in spintronics, nanoelectronics and optoelectronics applications. To this end a

dM

novel nanocomposite was produced having quantum confined Ge ad atoms on the rGO sheets. In the synthesis of graphene-based nanocomposites, it is important to obtain a homogeneous stable solution consisting of the precursor materials [33,34]. The approaches geared towards achieving high graphene dispersion in precursor solutions include usage of dispersing agents like surfactants and polymers or using graphene oxide (GO), which exhibits high dispersion

pte

[25]. GO is rich in reactive oxygen functional groups, such as carboxylic groups (-COOH) at the edges, epoxy (-C-O-C-) and hydroxyl groups (-OH) lying above and below the basal planes; these provide reactive sites or nucleation sites for heteroatom doping [16,25,35]. The carbon atoms in

ce

the epoxy and hydroxyl groups are covalently bonded to the oxygen atoms in a sp3 hybridization system; this forms a large fraction (0.5-0.6) of C in GO [36]. The carbon in the carboxyl and carbonyl groups are sp2-hybridized and are bonded to either carbon or oxygen atoms within these groups [37]. The atomic and electronic structure of GO, which is composed of a 2-D network of

Ac

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

Page 4 of 38

sp2 and sp3-bonded carbon atoms, allows for new functionalities [38]. Additionally, GO affords

4

Page 5 of 38

pt

simple, cost-effective and mass production of graphene-based materials [39,40]. Graphene or GO can be functionalized by different chemical routes which allow for covalent or non-covalent

us cri

coupling [35]. Herein, GO reduction and formation of rGO-GeO2 nanocomposite was achieved in a one-step microwave-assisted solvothermal synthesis.

In this synthesis, rGO prevents the

aggregation of Ge/GeO2 nanoparticles. The abundant oxygen functional groups of GO provide sites for adsorption of Ge ions. 2. Experimental section

an

2.1. Materials

Graphite powder and germanium ICP standard solution (1000 μg ml-1) were purchased from

dM

Sigma-Aldrich, Germany. Germanium tetrachloride was purchased from Alfa-Aesar, Germany. These reagents were of analytical grade and were used without further purification. 2.2. Synthesis of reduced graphene oxide-germanium quantum dot nanocomposite Graphene oxide (GO) was synthesized by oxidation and exfoliation of natural graphite using the

pte

modified Hummers method [41,42]. Reduced graphene oxide-germanium oxide (rGO-GeO2) nanocomposites were synthesized by microwave-assisted solvothermal reaction as reported previously in literature [6]. Typically, in the synthesis, 0.5 g of GO in 50 ml of anhydrous ethanol

ce

was ultrasonicated for 40 minutes to obtain a homogeneous suspension. 1.0 g of GeCl4 was mixed with 5 ml of anhydrous ethanol and stirred for 5 minutes; this was then added to the obtained GOethanol solution and stirred for 20 minutes. Finally, 20 ml of the as-obtained mixed solution was transferred into a 35 ml microwave vial and irradiated with microwaves at 180 °C for 20 minutes.

Ac

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

AUTHOR SUBMITTED MANUSCRIPT - NANO-115161.R1

The mass of GeCl4 was varied to obtain composites of different Ge content. After the solvothermal

5

AUTHOR SUBMITTED MANUSCRIPT - NANO-115161.R1

pt

reaction, a black colloidal suspension consisting of rGO-GeO2 was formed. This was washed thoroughly using ethanol and double distilled water and then dried overnight in a vacuum oven at

us cri

50 °C. The as-obtained rGO-GeO2 was thermally reduced at 650 °C for 2 h at a heating rate of 5 °C min-1 under 10% hydrogen-argon gas flow. 2.3. Characterization

Structural and morphological characterization of the nanocomposites was done using field emission scanning electron microscope (SEM: JEOL JSM 6100), transmission electron

an

microscope (TEM: JEOL JEM 1010) and a high-resolution transmission electron microscope (HRTEM: JEOL JEM 2100). Germanium concentration in the nanocomposites was determined from the inductively coupled plasma-optical emission spectroscopy (ICP-OES) on a Pekin Elmer

dM

Optima 5300 DV. The XRD spectra of the samples was obtained using a Bruker D8 Advance Xray powder diffractometer with a high-intensity Cu Kα radiation (λ = 0.15406 nm). Raman analysis of the samples was carried out using a DeltaNu Advantage 532™ Raman spectrometer. The thermal stability of the composites was determined using a Q series TM Thermal Analyzer DSC/TGA (Q600). The surface area of the nanocomposites was obtained using a Micrometrics

pte

TriStar II surface area and porosity analyzer. The infrared (IR) spectra of the samples were obtained using a Perkin Elmer spectrum 100 FTIR spectrometer with universal ATR sampling accessory.

The electrical conductivity was determined from four-point probe resistivity

ce

measurements. The UV-Vis absorption spectra and photoluminescence emission spectra were obtained from a Shimadzu UV-Vis-NIR spectrophotometer and a Perkin Elmer LS 55 fluorescence spectrometer, respectively. The magnetic measurements for the samples were done at room

Ac

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

Page 6 of 38

temperature using a vibrating sample magnetometer (VSM: Lakeshore 735) in applied magnetic fields of up to 1.5 kOe. 6

3. Results and discussion

us cri

3.1. Germanium concentration

The germanium content in the nanocomposites was quantified using ICP-OES. Both the rGOGeO2 and the rGO-Ge nanocomposites were digested by heating in 70% HNO3. It has been reported previously that Ge dissolves in HNO3 [43]. Reference stock solution was prepared from 1000 μg ml-1 germanium ICP standard solution. The concentration of Ge in the nanocomposites

an

as deduced by this procedure is given in Table 1.

Table 1. Germanium concentration in the nanocomposites.

Germanium concentration (%)

rGO-Ge-1 (rGO-GeO2-1)

0.25 (0.16)

dM

Sample

rGO-Ge-2 (rGO-GeO2-2)

0.51 (0.20)

rGO-Ge-3 (rGO-GeO2-3)

0.59 (0.23)

3.2. Morphology and structural properties

pte

The TEM and SEM micrographs of GO and the nanocomposites are shown in Figure 1 and Figure 2, respectively. The observed images of the composites show a multilayered structure with edges which scroll and fold slightly. The SEM image of GO show several layers stacked together while

ce

those of the composites manifest folded sheets which tend to crumple. The TEM images manifest a typical 2-D morphology of GO and the nanocomposites.

Ac

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

AUTHOR SUBMITTED MANUSCRIPT - NANO-115161.R1

pt

Page 7 of 38

7

us cri an dM

ce

pte

Figure 1. TEM image of (a) GO, (b) rGO-GeO2-1, (c) rGO-GeO2-2 and (d) rGO-Ge-2.

Ac

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

Page 8 of 38

pt

AUTHOR SUBMITTED MANUSCRIPT - NANO-115161.R1

8

us cri an dM

Figure 2. SEM image of (a) GO, (b) rGO-GeO2-2, (c) rGO-GeO2-3 and (d) rGO-Ge-2. Uniformly distributed Ge nanoparticles are anchored on the basal planes of rGO sheets as

pte

observed in the micrographs (Figure 3). The uniform distribution of the Ge nanoparticles on the rGO sheets attribute the nanoparticles growth to in situ reduction and self-assembly growth mechanism [25]. The particle size distribution of the spherical Ge nanoparticles is also shown in

ce

Figure 3. The diameters of the spherical Ge nanoparticles, for all the nanocomposites, range between 1.6- 9.0 nm. This is smaller than the Ge Bohr exciton radius (24.3 nm) [21] hence, the nanoparticles formed are Ge quantum dots. As observed in the histograms, rGO-GeO2-2 consist of large-sized particles relative to rGO-GeO2-1 and rGO-GeO2-3. Figure 4(a) manifests a well-

Ac

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

AUTHOR SUBMITTED MANUSCRIPT - NANO-115161.R1

pt

Page 9 of 38

resolved lattice structure with a d spacing of 0.32 nm assigned to the (311) plane of cubic

9

AUTHOR SUBMITTED MANUSCRIPT - NANO-115161.R1

pt

germanium. Similarly, lattice fringes with d-spacing values of 0.22 and 0.25 nm, corresponding to the (220) and (201) crystal planes of cubic Ge, respectively, were observed in the rGO-GeO2 The selected-area electron

us cri

samples as typified in the HR-TEM micrograph (Figure 4(b)).

diffraction (SAED) pattern shown in Figure 4(c) further confirms the high crystalline structure of the samples. The d-spacing values of 0.33, 0.22 and 0.17 nm determined from the rings pattern are consistent with the (111), (220) and (311) planes of Ge diamond cubic phase, respectively. Therefore, the crystalline Ge nanoparticles have the typical diamond structure. Moreover, the

dM

an

observed ring pattern show that the nanoparticles are polycrystalline in nature.

pte

Figure 3. HR-TEM images of (a) rGO-GeO2-1, (b) rGO-GeO2-2 and (c) rGO-GeO2-3. Inset is

ce

the particle size distribution histograms of the Ge quantum dots.

Ac

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

Page 10 of 38

10

Page 11 of 38

area electron diffraction (SAED) pattern of rGO-Ge-2.

pt

Figure 4. (a) HR-TEM image of rGO-Ge-2, (b) HR-TEM image of rGO-GeO2-3 and (c) selected

us cri

To identify the crystalline phases of the composites, powder XRD was performed for the samples. The XRD patterns of the nanocomposites are shown in Figure 5. The Bragg reflection angles for rGO-Ge occurring at 26°, 46° and 56°are assigned to the (111), (220) and (311) planes of diamond cubic phase Ge, respectively (JCPDS card no. 04-0545). The reflection at 26° overlap with that of (002) plane of rGO. The reflections at 35° and 39° correspond to the (110) and (111)

an

planes of GeO2; an indication that the GeO2 is not completely reduced by the thermal reduction treatment. This explains the weight loss of the sample at 720 °C in the TGA thermogram (Figure 6). For rGO-GeO2, the Bragg reflections which occur at 21° and 42.5° correspond to the (100) and

dM

(200) planes, respectively, of the hexagonal phase of GeO2 (JCPDS card no. 8052), while the reflection at 31° can be indexed to the (201) plane of Ge (JCPDS 72-1089). The Bragg reflection of GeO2 phase at 42.5° overlap with that of (220) plane of Ge. Therefore, the XRD patterns not only confirms the high crystalline quality of the Ge and GeO2 nanoparticles but also their polycrystalline nature. Both the HR-TEM and XRD analysis revealed the existence of Ge crystal

pte

planes in the rGO-GeO2 composites; this indicates the occurrence of partial reduction of the GeO2

ce

nanoparticles during the microwave synthesis.

Ac

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

AUTHOR SUBMITTED MANUSCRIPT - NANO-115161.R1

11

AUTHOR SUBMITTED MANUSCRIPT - NANO-115161.R1

pt

(a) C (001) GeO2 (100) Ge (201)

(b)

C (002)/Ge (111)

0

10

20

30

40

50

2 theta

an

GeO2 (110) GeO (111) 2 C (100) Ge (220) Ge (311)

us cri

Intensity (a.u.)

Ge (220)/GeO2 (200)

60

70

80

90

3.3. Thermal stability

dM

Figure 5. XRD patterns for (a) rGO-GeO2-2 and (b) rGO-Ge-2.

Thermogravimetric analysis (TGA) for the samples was carried out in nitrogen gas atmosphere. The nanocomposites were heated from room temperature to 1000 °C at a rate of 10 °C/min; the obtained TGA profiles are shown in Figure 6. The nanocomposites exhibited weight loss steps

pte

between 50-150 °C and at 200 °C due to water loss [44] and pyrolysis of the oxygen functional [45], respectively.

The weight further decreased slowly up to about 720 °C due to the

decomposition of GeO2. The decomposition of rGO-Ge sample at 400 °C and 720 °C corroborates

ce

with its XRD pattern (Figure 5) which shows two peaks assigned to GeO2 nanoparticles. Exothermic peaks at 120 °C, 200 °C, 400 °C and 720 °C accompanied the weight loss of the samples as shown in the TGA-DTA curves of Figure 6. Generally, the rGO-Ge composite is observed to be more thermally stable than the rGO-GeO2 composite.

Ac

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

Page 12 of 38

12

us cri an dM

Figure 6. TGA thermograms and derivative thermograms of the nanocomposites.

pte

3.4. Surface chemistry

Fourier transform infrared (FTIR) spectrometer was used to obtain the infrared (IR) spectrum of GO and the composites shown in Figure 7. The IR spectra of GO show the presence of different oxygen functional groups. The O-H stretching vibration of the hydroxyl group represented by a

ce

wide peak between 3200-3400 cm-1, C-O stretching vibration of the epoxide group at 1040 cm-1, C=O stretching vibration typical of the aliphatic ketone of the carbonyl group at 1618 cm-1 and 1720 cm-1, C-OH stretching vibration at 1220 cm-1, and O-H deformation of the C-OH groups at

Ac

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

AUTHOR SUBMITTED MANUSCRIPT - NANO-115161.R1

pt

Page 13 of 38

1400 cm-1 [46-48]. These oxygen functional groups provide sites for anchoring Ge nanoparticles. In the rGO-GeO2 spectra, the peaks at 1720 cm-1 and 1060 cm-1 show the presence of the C=O and 13

AUTHOR SUBMITTED MANUSCRIPT - NANO-115161.R1

pt

C-O stretching vibrations, respectively. The vanishing of the O-H indicates both the reduction of GO and the formation of C-O-Ge bonds, following the substitution of hydrogen ions by Ge ions

us cri

[6,49]. The functionalization achieved at the epoxy and hydroxyl groups of GO was also confirmed by the HR-TEM images which showed Ge nanoparticles anchored on the basal planes of the rGO sheets. After the carbonization process, the only remaining observable peak is that for the C=O stretching vibration at 1683 cm-1, though it is greatly reduced in intensity. The C=O stretching vibration of GO is known to facilitate the attachment of nanoparticles via covalent bonding/electrostatic coupling [50].

an

Low-intensity peaks occurring at 3400 cm-1, 850 cm-1 and 750 cm-1 in the rGO-GeO2 composites are characteristic of the GeO-H, Ge-O and Ge-OH stretching vibrations, respectively

dM

[25,51]. The 850 cm-1 peak for Ge-O stretching vibration is also present in the rGO-Ge sample. Its presence corroborates with the samples XRD pattern and TGA-DTA profile. The hexagonal crystalline phase of GeO2 is usually identified by a triplet at 530 cm-1, 560 cm-1 and 590 cm-1 [52]. A peak of low intensity for this triplet is observed in the rGO-GeO2 spectra though it is not split. Whereas there is a marked distinction between the rGO-GeO2 and rGO-Ge IR spectra, there are

pte

peaks that are common to both composites as follows. The C=C stretching vibration peak at 1580 cm-1 imply a π-π stacking between the Ge nanoparticles and the sp2 electrons network of rGO [35]. It also indicates a restoration of the long range conjugated graphene structure [53]. The peaks at

ce

2100 cm-1 and 2327 cm-1 are assigned to the C≡C stretching vibration; these peaks are attributed to a strong hybridization of the graphene π-bands with Ge d-bands. On the other hand, those at 2700 cm-1 and 2900 cm-1 are assigned to the C-H stretching vibration. The peak at 1460 cm-1, assigned to the scissoring and symmetric bending vibrations of the Ge-C bond, manifest that the

Ac

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

Page 14 of 38

Ge nanoparticles are attached to rGO via covalent bonding [31]. The absorption frequency of the

14

Page 15 of 38

pt

C-H vibration attributes the bond to sp3 hybridization implying that the formation of the Ge-C bond is associated with the substitution of the sp2 bonds by sp3 bonds.

Hence, in the

covalent interactions.

us cri

nanocomposites, the Ge/GeO2 nanoparticles are attached to the rGO via both covalent and nonThe strong chemical interaction between rGO and the Ge/GeO2

nanoparticles is attributed to the fact that both C and Ge atoms are of the same group in the periodic table and hence, are isovalent [54]. Moreover, the Ge ions which are positive are easily absorbed by the negative ions of GO via electrostatic interaction [41].

an

(a)

C-O

C=O

(b)

Ge-OH

dM

Transmittance (a.u.)

O-H

C-H

C-H

3500

3000

GeO-H

Ge-O

C-O

C=C

2500

2000

Ge-C Ge-O

1500

1000 -1

Wavenumber (cm )

ce

4000

Ge-C

C=O C=C

pte

(c)

Figure 7. FTIR spectra of (a) GO, (b) rGO-GeO2-2 and (c) rGO-Ge-2.

Ac

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

AUTHOR SUBMITTED MANUSCRIPT - NANO-115161.R1

3.5. Raman analysis

15

500

0

AUTHOR SUBMITTED MANUSCRIPT - NANO-115161.R1

pt

Raman spectroscopy was employed to investigate the graphitic nature and the defect state of the samples. The acquired Raman spectra of the samples are shown in Figure 8. The GO spectrum

us cri

exhibit peaks at 1345 cm-1 and 1585 cm-1 corresponding to the D-band and G-band, respectively.

The D-band is characteristic of the defects and disorders within the graphitic structure whereas the G-band is related to the stretching vibration of the sp2 hybridized carbon atoms. Table 2 shows the shifts in the position of both the D and G-bands peaks in the spectra of rGO-GeO2 and rGOGe composites. The observed blue shifts of the G peak signify an occurrence of charge transfer between the Ge nanoparticles and the rGO sheets [55]. This indicates that the Ge nanoparticles

an

are not just physically attached to the rGO sheets but interact with the carbon atoms via bond formation [49]. The crystallinity of carbon-based samples is usually deduced from the Tuinstra-

dM

Koening relationship, ID/IG, which is the ratio of the integrated area of the D-band to the integrated area of the G-band [56]. Conventionally, highly crystalline samples exhibit low values of ID/IG and vice versa. As shown in Table 2, the ID/IG value of the rGO-GeO2 samples increase with an increase in the Ge content in the samples implying that the incorporation of the Ge ad atoms is associated with the creation of defects in the graphitic structure. However, the thermally reduced

pte

sample manifest the lowest ID/IG value. This is attributed to the elimination of disordered amorphous carbon during the thermal reduction treatment hence, yielding a graphitic structure of

ce

high crystallinity.

Ac

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

Page 16 of 38

16

us cri

an

Figure 8. Raman spectra of GO and the nanocomposites. Table 2. Raman analysis of GO and the nanocomposites. D band (cm-1)

G band (cm-1)

ID/IG

1345

1585

0.77

1341

1589

0.82

1342

1594

1.05

1348

1604

1.32

1341

1587

0.70

GO rGO-GeO2-1 rGO-GeO2-2 rGO-GeO2-3

pte

rGO-Ge-2

dM

Sample

3.6. Textural properties

The surface area and porosity of the nanocomposites listed in Table 3 were measured by the

ce

nitrogen gas absorption Brunauer-Emmett-Teller (BET) method. The surface area and porosity of the composites increased with the increase in Ge/GeO2 content. This trend is consistent with the surface morphology observed in the SEM images. However, comparing the surface area and porosity of rGO-GeO2-2 and rGO-Ge-2, it is noted that the surface area reduces whilst the pore

Ac

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

AUTHOR SUBMITTED MANUSCRIPT - NANO-115161.R1

pt

Page 17 of 38

size increases to more than twice after the thermal reduction treatment. This is attributed to the 17

AUTHOR SUBMITTED MANUSCRIPT - NANO-115161.R1

pt

structural morphology consisting of more folded sheets which tend to crumple as shown in the SEM micrograph. It is inferred that the Ge nanoparticles which are defects on the rGO surface

us cri

combined with holes and/or defects resulting from the evolution of CO2 and CO during the thermal reduction treatment aid the folding and crumpling of the rGO sheets because of their inherent

smaller mechanical resistivity [57]. The porous structure of the composites, therefore, is enhanced by the increased Ge/GeO2 content and the thermal reduction treatment.

The adsorption isotherms of the composites are shown in Figure 9. The observed isotherms are typically the type IV based on the international union of pure and applied chemistry

an

classification guidelines [58] hence, the samples are classified as type IV mesoporous materials. rGO-GeO2-2 and rGO-GeO2-3 exhibited H1 type of hysteresis loops between 0.7-1.0 P/Po; such a

dM

type of loop is inherent in materials with regular spherical-shaped pores. On the contrary, rGOGeO2-1 and rGO-Ge-2 exhibited an H3 type hysteresis loops between ≈ 0.45-1.0 P/Po with a cavitation at ≈ 0.5 P/Po. This type of hysteresis loop is characteristic of materials consisting of plate-like particles with slit-shaped pores. The observed hysteresis loops are consistent with the samples morphological structures observed in the SEM images.

Surface area

Pore volume

Pore size

(m2 g-1)

(cm3 g-1)

(nm)

31

0.093

8.69

ce

Sample

pte

Table 3. Surface area and porosity of the composites.

rGO-GeO2-1 rGO-GeO2-2

88

0.190

7.35

rGO-GeO2-3

152

0.221

7.69

rGO-Ge-2

52

0.222

15.94

Ac

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

Page 18 of 38

18

us cri an dM

Figure 9. N2 adsorption isotherms of the composites. 3.7. Electrical conductivity

pte

Four-point probe measurements were carried out on pellets of 0.2 mm thickness formed from 0.03 g of the nanocomposites; the results are as shown in Table 4. The measurements were done on pellets formed from the as obtained composites and thermally reduced samples at 200 °C for two

ce

hours under 10% hydrogen-argon gas flow. The electrical conductivity was observed to increase with an increase in Ge/GeO2 content in the nanocomposites. However, after the thermal reduction treatment, rGO-Ge-2 sample exhibited the highest electrical conductivity. The high electrical conductivity in the composites is attributed to two factors: Firstly, is the restoration of the

Ac

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

AUTHOR SUBMITTED MANUSCRIPT - NANO-115161.R1

pt

Page 19 of 38

graphene π-electrons conjugated network leading to the formation of more percolation pathways

19

AUTHOR SUBMITTED MANUSCRIPT - NANO-115161.R1

pt

within the sp2 carbon atoms; secondly, is the interaction between Ge/GeO2 nanoparticles and rGO via π-π stacking and covalent bonding, as shown in the IR spectra, which leads to graphene π-

us cri

bands and Ge d-bands hybridization, hence, leading to an increased delocalized electron system.

That is, charge transfer via hopping occurs from Ge to rGO and vice versa in the composites. The electrical conductivity is enhanced in the composites after the thermal reduction treatment because of the increase in the conjugation of the sp2 domains of rGO and reduction of GeO2; consequently, increased π-π stacking of the Ge nanoparticles and rGO sp2 domains occurs in the composites The carbonization process reduces GeO2 nanoparticles to Ge, as shown in the XRD pattern of Figure

an

10a.

To further elucidate the electronic transport in the nanocomposites, the effect of the Ge ad

dM

atoms on the graphene band structure is considered. 2-D graphene is known to have zero density of states (DOS) at the EF (which is at the Dirac point for pristine graphene), with its π and π* bands crossing linearly at the EF. In contrast, GO has finite energy band gap which close in upon reduction, as the conjugation of the sp2 electron network is restored. The interaction between rGO and Ge yield specialized bands around the EF; these are derived from the combination of Ge pz

pte

orbitals with π* conduction band states of rGO through the back donation of electrons from Ge and sp2 carbon network [54]. These bands give rise to a finite density of states near the EF which results in the observed high electrical conductivity.

Moreover, because of the quantum

ce

confinement effects in the Ge quantum dots, the density of states is enhanced since the number of the discrete electron energy levels in a system is inversely proportional to the system’s dimension. Considering the combined effect of the quantized electron and hole wave functions of Ge quantum dots and the structural configuration of rGO, it is inferred that finite energy band gaps are created

Ac

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

Page 20 of 38

within the system making the composites semiconductors.

20

Page 21 of 38

Bulk resistivity

Electrical conductivity

concentration (%)

(Ω sq-1)

(Ω cm)

(S cm-1)

pt

rGO-GeO2-1

0.16

10367

207.34

0.005

rGO-Ge-1

0.25

39

0.81

1.238

rGO-GeO2-2

0.20

1233

24.66

0.041

rGO-Ge-2

0.51

16

0.34

3.049

rGO-GeO2-3

0.23

831

16.62

0.060

rGO-Ge-3

0.59

Germanium

26

0.53

1.872

pte

dM

Sample

us cri

Sheet resistance

an

Table 4. Electrical conductivity of the nanocomposites.

ce

Figure 10. (a) XRD pattern and (b) IR spectrum of rGO-Ge-3 reduced at 200 °C.

3.8. Optical properties

Ac

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

AUTHOR SUBMITTED MANUSCRIPT - NANO-115161.R1

The UV-Vis absorption spectra and the photoluminescence (PL) emission spectra were obtained from aqueous dispersions of the samples in ethanol. The nanocomposites exhibit a strong 21

AUTHOR SUBMITTED MANUSCRIPT - NANO-115161.R1

pt

absorption in the UV region with peaks at 220 nm for all samples, 271 nm for rGO-GeO2-1 and 287 nm for rGO-GeO2-2 and rGO-GeO2-3 as shown in Figure 11. Similarly, a tail spanning from

us cri

400 nm is observed in the spectra of rGO-GeO2-2 and rGO-GeO2-3. The observed peaks at 271

nm and 287 nm in the nanocomposite spectra are due to π→π* transitions of the double and triple bonds (as observed in the IR spectra) which are basically charge transfer transitions (from Ge to rGO and vice versa). This manifests a restoration of the conjugated electron network of graphene [59] and the hybridization of the graphene π-bands and Ge d-bands. Because a conjugated electron network of graphene and a π-π stacking between carbon and Ge atoms implies a delocalized

an

electron network hence, a decrease in the energy for electronic transitions. The absorption peak at 220 nm results from π→π* transitions of the aromatic C-C bond [47,60]. The rGO-GeO2-2

dM

spectrum is slightly different from the other nanocomposite spectra in that its 287 nm peak is broadened and it has an additional inconspicuous peak at 360 nm. This is attributed to its largesized particles within a wide distribution range in comparison to the other composites. Similarly, the 287 nm peak of rGO-GeO2-1 is blue shifted to 271 nm because its dominated by smaller sized particles relative to the other samples [54,61]. The pronounced and well-defined 271 nm peak in

pte

the rGO-GeO2-1 spectra suggests that smaller sized particles favor the electronic transitions of narrow energy distribution. The blue shift in this peak is ascribed to a relatively low density of molecular orbitals in the conduction band since the sample is dominated by small sized particles

ce

[62]. These unique observations in the rGO-GeO2-1 and rGO-GeO2-2 spectra coupled with the observed rising absorption edge from 400 nm are due to quantum confinement effects [63]. Moreover, the absorption intensity increases with the increase in the Ge/GeO2 content in the nanocomposites because of increased charge transfer transitions.

Ac

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

Page 22 of 38

22

0.6

rGO-GeO2-1 rGO-GeO2-3

0.4

0.2

0.0 300

400

500

600

700

an

Wavelength (nm)

us cri

Intensity (a.u.)

rGO-GeO2-2

Figure 11. UV-Vis absorption spectra of the nanocomposites.

Figure 12 shows the Tauc plots from the samples absorption data. The composites manifest

dM

two absorption edges corresponding to rGO and Ge structures. The optical band gaps determined from these plots are: 4.13 eV and 5.38 eV for rGO-GeO2-1; 3.0 eV and 5.0 eV for rGO-GeO2-2; and 3.5 eV and 5.15 eV for rGO-GeO2-3. The optical band gap is observed to depend on the Ge particle size i.e. the sample dominated by large sized Ge nanoparticles has the lowest optical band

pte

gap value. This corroborates with a previous study in which the optical band gap was observed to increase with a decrease in the quantum dots size [64]. The dependence of the optical band gap value on the Ge nanoparticles size is ascribed to quantum confinement effects whereby the energy level of the spherical Ge quantum dots first excited state increases as the particle size decrease

ce

[65]. Thus, the determined optical band gap values evince this fact since, the obtained absorption spectra result from electronic transitions from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO).

Ac

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

AUTHOR SUBMITTED MANUSCRIPT - NANO-115161.R1

pt

Page 23 of 38

23

us cri

an

Figure 12. Tauc plots from UV-Vis analysis for the nanocomposites.

The photoluminescence spectra of the nanocomposites at an excitation wavelength of 340 nm are shown in Figure 13(a). All the nanocomposites exhibited luminescence in the near UV and

dM

the visible region with a narrow peak of high intensity at 380 nm and a broad one of relatively low intensity at 450 nm as well as a tail spanning to the visible region. These narrow peaks in the PL spectra are attributed to the radiative electron-hole recombination in the localized sp2 clusters within which electronic coupling is negligible. On the other hand, the broader peaks of lower

pte

intensity result from two contributions: First, is the delocalized sp2 domains of rGO in which the excitons transport to non-radiative recombination centers thereby quenching the luminescence; second, is energy transfer from rGO to Ge nanoparticles and vice versa as the two interact via π-π stacking and covalent bonding. This energy transfer is ascribed to two possible mechanisms: (i)

ce

is the quantum mechanical interaction from a possible overlap of the electronic wave functions of rGO and Ge quantum dots, because the confinement of the wave functions of Ge holes and electrons in real space leads to a spread of their wave functions in the momentum space; (ii), is the

Ac

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

Page 24 of 38

pt

AUTHOR SUBMITTED MANUSCRIPT - NANO-115161.R1

resonant energy transfer whereby there are high chances of the exciton being transported to quenching centers hence, dissipating energy via non-radiative process. 24

This hypothesis is

Page 25 of 38

pt

supported by the fact that the 450 nm peak in the rGO-GeO2-1 spectrum is more intense than those of the other composites even though the sample has the least Ge content. Per this hypothesis, it

us cri

would be expected that the rGO-GeO2-3 450 nm peak would exhibit the least intensity, but instead,

rGO-GeO2-2 450 nm peak has the least intensity evidencing quantum confinement effects. That is, nanoparticles of a specific size absorb and emit light at their respective characteristic wavelengths, such that larger sized particles are usually characterized by emissions of weak intensity at larger wavelengths [31,66]. Therefore, the PL spectrum of rGO-GeO2-2, like in the UV-Vis spectrum, is different from the other composites spectra.

It has a broadened and

an

suppressed peak at 450 nm attributed to its larger particle sizes supporting quantum confinement effects. The 380 nm emission peak is absent in the rGO-Ge sample reduced at 200 °C indicating

dM

a successful reduction after the thermal reduction treatment; this agrees with the observed XRD pattern and IR spectra of the sample (Figure 10).

The peak at 450 nm is inhomogeneous in breadth for all the samples because of the continuous distribution of the Ge particles sizes. A direct manifestation of quantum confinement effects in samples with a continuous particle size distribution is a monotonic shift of the emission

pte

wavelength with the change in the excitation wavelength [61]. Figure 13(b) shows the PL emission spectra of the rGO-Ge-3 (reduced at 200 °C) at excitation wavelengths in the range of 340-370 nm. The emission peaks exhibit monotonic blue shift as the excitation wavelength is increased;

ce

this further confirms quantum confinement effects in the Ge nanoparticles. The PL intensity is observed to decrease as the excitation wavelength is increased while the emission peak widens for increased excitation wavelength. The wavelength dependence of the PL intensity and emission peak breadth is attributed to the Ge quantum dots polydispersity.

Ac

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

AUTHOR SUBMITTED MANUSCRIPT - NANO-115161.R1

Wherein, smaller sized

nanoparticles, exhibiting high PL intensity, are selectively excited at shorter wavelengths than the

25

AUTHOR SUBMITTED MANUSCRIPT - NANO-115161.R1

pt

larger particles. This corroborates with the particle diameter histogram (inset of Figure3 (c)) which

an

us cri

shows a high percentage of small-sized nanoparticles.

Figure 13. Photoluminescence spectra of the nanocomposites (a) and rGO-Ge-3 reduced at 200

dM

°C for different excitation wavelengths (b). 3.9. Magnetic properties

Magnetization measurements for GO and the nanocomposites was done at room temperature (~295 K) under an applied magnetic field of up to 1.5 kOe. A rGO-650 sample, obtained by thermal

pte

reduction of GO at 650 °C, was also analysed for comparison purposes. The obtained hysteresis loops are shown in Figures. 14 and 15. The corresponding coercivity (HC), magnetization (M) and the loops squareness as deduced from the hysteresis loops are listed in Table 5. The observed hysteresis loops for all the samples are typical of paramagnetic materials. The magnetization in

ce

GO is attributed to the oxygen functional groups (defects) as shown in the FTIR spectrum (Figure 7). Since GO is electrically insulating, its magnetism is explained based on the Hubbard type model in real space [67]. The π-electron system of graphene is considered as half-filled as each

Ac

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

Page 26 of 38

of the sp2 carbon atoms donates one pz-orbital and one π-electron [14]. For such a half-filled system, short-range antiferromagnetic ordering has been shown to arise from vacancy or 26

Page 27 of 38

pt

chemisorbed defects based on the Hubbard model [14,68]. The net magnetic moment within the short-range antiferromagnetic cluster is given by 𝑀~(𝑁𝐴 − 𝑁𝐵 ), where NA and NB are the number

us cri

of A and B sublattice points, respectively [67]. A vacancy defect or the OH group of GO can cause

an imbalance in NA and NB, yielding a local magnetic moment [68]. The OH clusters are usually associated with magnetic moments of higher spin states at higher spin densities, relative to the other oxygen functional groups of GO, because of its high magnetic inducing efficiency [69,70]. Appreciable increase in magnetization of GO occurs upon its thermal reduction at 650 ºC. Thermal reduction of GO has been associated with the disintegration of GO into fragments with exposed

an

zigzag edges [71]. Since, the rGO-650 sample exhibited reduced ID/IG ratio of 0.64, it is inferred that the enhanced magnetization follows from the exposed zigzag edges. Also, the reduction of

dM

strains and topological defects in the graphitic structure during the removal of oxygen are a possible additional cause of the observed enhancement. The magnetization of the nanocomposites was observed to increase with the increase in Ge concentration. Also, the magnetization is remarkably enhanced after the thermal reduction treatment. The magnetization in the nanocomposites is attributed to the Ge adatoms which create

pte

an imbalance between the two sublattices of graphene, resulting in quasi-localized states near the EF. When these states are singly occupied, because of Coulomb repulsion, they yield magnetic moments [69]. The slight drop in the magnetization value of rGO-GeO2-3, the sample with the

ce

highest concentration of Ge adatoms, is ascribed to two factors: (i) beyond an optimum Ge adatom concentration, there is a possibility of double occupation of some of the generated quasi-localized states [72]; (ii) the size dependence of magnetization, whereby large sized particles are associated with higher magnetic moments [73] and in this case, the Ge quantum dots of rGO-GeO2-2 are

Ac

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

AUTHOR SUBMITTED MANUSCRIPT - NANO-115161.R1

larger in diameter relative to rGO-GeO2-3. The FTIR spectra of the nanocomposites (Figure 7)

27

AUTHOR SUBMITTED MANUSCRIPT - NANO-115161.R1

pt

show the C-H stretching vibration peaks at 2700 cm-1 and 2900 cm-1, which attributes the bond to sp3 hybridization. This indicates the presence of sp3-type defects. The sp3-type defects created on

us cri

the basal planes of graphene are associated with magnetic moments of higher spin densities [69,74]. These effects are responsible for the enhanced magnetization upon incorporation of Ge adatoms on the graphene lattice. Also, significant increment in magnetization is observed upon

the reduction of rGO-GeO2 to rGO-Ge. This is attributed to more singly occupied quasi-localized states generated as the GeO2 is reduced to Ge. The Raman analysis of the rGO-Ge composite showed improved crystallinity evidenced by its low ID/IG ratio. The improved crystallinity also

an

serves to enhance the magnetization. It is instructive to illustrate the evolution of magnetic moments based on the effect of the Ge adatoms on the band structure of graphene.

As

dM

aforementioned, specialized bands are generated around the EF upon incorporation of Ge on graphene lattice; the effect of these is perceived as a finite DOS at the EF [54]. The splitting of the spin-up and spin-down states of these bands yield a net magnetic moment in the supercell [54,75]. Charge transfer from the Ge adatom to rGO via hopping is responsible for the creation of the imbalance in the spin states of the bands.

pte

Coercivity of the nanocomposites was observed to increase with an increase in the Ge adatoms concentration and also after the thermal reduction. On the other hand, GO exhibited a high coercivity value, even higher than the lowest Ge concentration nanocomposite (rGO-GeO2-

ce

1). Coercivity is essentially a measure of a materials magnetocrystalline anisotropy. The coercive field is given by equation 1[67,71],

Ac

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

Page 28 of 38

𝐻𝐶 = 𝐾𝑉 ⁄𝑀

(1)

where K is the magnetic anisotropy energy density, V and M are the volume and the net magnetic moment of a single domain particle, respectively. The presence of wrinkles/crumpling in a 28

Page 29 of 38

pt

material structure leads to reduced average local magnetic anisotropy energy density and small long range magnetic interaction [67]. Therefore, the relatively high coercivity in GO is ascribed

us cri

to its less wrinkled structure which give rise to a high KV value. As depicted in the TEM and SEM micrographs, shown in Figures. 1 and 2, respectively, the GO structure has less wrinkles in comparison to the nanocomposites structure. Remarkable improvement in the coercivity of GO

follow from the thermal reduction process. GO is known to have finite non-zero energy band gaps due to the distortion of the graphene sp2 electron network but the thermal reduction would minimize these gaps. Hence, enabling coupling between the isolated magnetic moments which is

an

responsible for the improved coercivity. This coupling is mediated via the RKKY type interaction of graphene’s delocalized π-electrons [76]. For the nanocomposites, the coercivity is observed to

dM

increase with the increase in the Ge concentration and after the thermal reduction. This observation is attributed to the occurrence of long range indirect Ge-Ge coupling via the combination of Ge orbitals with the graphene π and π* band states leading to a higher spin polarization at EF [54]. For the thermally reduced composite, the enhanced coercivity is ascribed to relatively less wrinkled structure and a stronger long-range magnetic coupling mediated by the Ge adatoms

pte

following the reduction of GeO2. On the other hand, rGO-GeO2-2 would be expected to have a higher coercivity than rGO-GeO2-3 following their magnetization trend, but the converse is observed. This is attributed to its larger sized Ge quantum dots relative to the rGO-GeO2-3

ce

composite as the coercive field decreases with increased particle size [73].

Ac

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

AUTHOR SUBMITTED MANUSCRIPT - NANO-115161.R1

29

us cri

pte

dM

an

Figure 14. Magnetic hysteresis loops for (a) GO and (b) rGO-650.

Figure 15. Magnetic hysteresis loops for the nanocomposites. Table 5. Magnetic parameters of GO and the nanocomposites. Germanium

Magnetization

Coercivity

Squareness

concentration (%)

(M) (emu/g)

(HC) (Oe)

(× 10-3)

GO

0

0.84

112.53

17.08

rGO-650

0

2.59

245.00

34.89

0.16

3.81

102.22

17.03

ce

Sample

Ac

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

Page 30 of 38

pt

AUTHOR SUBMITTED MANUSCRIPT - NANO-115161.R1

rGO-GeO2-1

30

0.20

7.31

148.28

rGO-GeO2-3

0.23

6.73

155.63

rGO-Ge-2

0.51

21.81

168.62

37.69 19.78 26.47

us cri

rGO-GeO2-2

4. Conclusion

Reduced graphene oxide-germanium nanocomposites of high crystalline quality have been synthesized by the microwave-assisted solvothermal reaction in which GO reduction and

an

formation of rGO-GeO2 nanocomposites was achieved in one step. HR-TEM micrographs show that spherical shaped germanium quantum dots of diameter ranging between 1.6- 9.0 nm are

dM

anchored on the basal planes of rGO. The nanocomposites exhibited high electrical conductivity which increases with increase in the Ge content. The observed enhancement of the electrical properties of the composites after the thermal reduction treatment results from the increase in the conjugation of the sp2 domains of rGO and consequently increased π-π stacking of the Ge nanoparticles and rGO sp2 domains. Defect induced magnetic moments evolve in the composites

pte

as the Ge adatoms create an imbalance between the two sublattices of graphene resulting in quasilocalized states near the EF. The magnetization and coercivity of the composites showed a marked dependence on the Ge quantum dots size and concentration.

Our studies reveal that

ce

functionalization of graphene by heavy elements results in strong defect-induced magnetization. Furthermore, the high magnetization achieved in the nanocomposites advances graphene’s potential in magnetic-based technological applications.

Ac

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

AUTHOR SUBMITTED MANUSCRIPT - NANO-115161.R1

pt

Page 31 of 38

The nanocomposites exhibited quantum confinement effects as observed in the UV-Vis absorbance spectra and photoluminescence emission spectra. The samples manifested strong 31

AUTHOR SUBMITTED MANUSCRIPT - NANO-115161.R1

pt

absorption and luminescence in the UV and near UV regions, respectively, and a moderate luminescence in the visible region. The optical band gap values, as well as the PL intensity of the

us cri

nanocomposites, showed a marked dependence on the Ge quantum dots size. The moderate PL intensity is attributed to non-radiative electron-hole recombination and energy transfer mechanisms which come into play because of the quantum confinement effects. Thus, the nanocomposites synthesized herein are promising candidates for spintronic, electronic and optoelectronic applications. Moreover, the fact that the composites are readily dispersible in a

an

polar solvent makes them viable for cost-effective solution-based device processing. Acknowledgements

The authors thank the National Research Foundation (NRF), the University of KwaZulu-Natal and

dM

the UKZN Nanotechnology Platform for funding this research work. T.A. Amollo wishes to thank Dr T. Moyo and P.E. Itegbeyegone of Physics Department, the University of KwaZulu-Natal for assisting with the magnetization measurements. The authors declare no conflicts of interest. References

Geim A K, Novoselov K S 2007 The rise of graphene Nat. Mater. 6 183-91

[2]

Wang S, Ang P K, Wang Z, Tang A L L, Thong J T, Loh K P 2009 High mobility, printable,

pte

[1]

and solution-processed graphene electronics Nano Lett. 10 92-8 [3]

Candini A, et al 2017 High photoresponsivity in graphene nanoribbon field effect transistor devices contacted with graphene electrodes J. Phys. Chem. C 121 10620-25 De Sanctis A, Barnes M D, Amit I, Craciun M F, Russo S 2017 Functionalised hexagonal-

ce

[4]

domain graphene for position-sensitive photodetectors Nanotechnology 28 124004

[5]

Bonaccorso F, Sun Z, Hasan T, Ferrari A 2010 Graphene photonics and optoelectronics Nat. Photonics 4 611-22

Ac

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

Page 32 of 38

32

Page 33 of 38

Li D, Seng K H, Shi D, Chen Z, Liu H K, Guo Z 2013 A unique sandwich-structured

pt

[6]

C/Ge/graphene nanocomposite as an anode material for high power lithium ion batteries J.

[7]

us cri

Mater. Chem. A 1 14115-21

Landi G, Sorrentino A, Iannace S, Neitzert H 2016 Differences between graphene and graphene oxide in gelatin based systems for transient biodegradable energy storage applications Nanotechnology 28 054005

[8]

Choi W, Lahiri I, Seelaboyina R, Kang Y S 2010 Synthesis of graphene and its applications: A review Crit. Rev. Solid State Mater. Sci. 35 52-71

[9]

Andrei E Y, Li G, Du X 2012 Electronic properties of graphene: a perspective from

[10]

an

scanning tunneling microscopy and magnetotransport Rep. Prog. Phys. 75 056501

Amollo T A, Mola G T, Kirui M, Nyamori V O 2017 Graphene for thermoelectric applications: Prospects and challenges Crit. Rev. Solid State Mater. Sci. 1-25

[11]

Novoselov K S, Geim A K, Morozov S V, Jiang D, Zhang Y, Dubonos S V, Grigorieva I

[12

dM

V, Firsov A A 2004 Electric field effect in atomically thin carbon films Science 306 666-9 Tiwari S K, Kumar V, Huczko A, Oraon R, De Adhikari A, Nayak G 2016 Magical allotropes of carbon: Prospects and applications Crit. Rev. Solid State Mater. Sci. 1-61 [13]

Tombros N, Jozsa C, Popinciuc M, Jonkman H T, Van Wees B J 2007 Electronic spin transport and spin precession in single graphene layers at room temperature arXiv preprint arXiv:0706.1948

Yazyev O V 2010 Emergence of magnetism in graphene materials and nanostructures Rep.

pte

[14]

Prog. Phys. 73 056501 [15]

Boukhvalov D, Katsnelson M 2009 Chemical functionalization of graphene J. Phys.: Condens. Matter 21 344205

[16]

Wang X, Sun G, Routh P, Kim D-H, Huang W, Chen P 2014 Heteroatom-doped graphene

ce

materials: syntheses, properties and applications Chem. Soc. Rev. 43 7067-98

[17]

Fan M, Feng Z-Q, Zhu C, Chen X, Chen C, Yang J, Sun D 2016 Recent progress in 2D or 3D N-doped graphene synthesis and the characterizations, properties, and modulations of N species J. Mater. Sci. 51 10323-49

Ac

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

AUTHOR SUBMITTED MANUSCRIPT - NANO-115161.R1

[18]

González-Herrero H, et al 2016 Atomic-scale control of graphene magnetism by using hydrogen atoms Science 352 437-41 33

AUTHOR SUBMITTED MANUSCRIPT - NANO-115161.R1

Esquinazi P, Hergert W, Spemann D, Setzer A, Ernst A 2013 Defect-induced magnetism

pt

[19]

in solids IEEE Tran. Magn. 49 4668-74

Kamata Y 2008 High-k/Ge MOSFETs for future nanoelectronics Mater. Today 11 30-8

[21]

Maeda Y 1995 Visible photoluminescence from nanocrystallite Ge embedded in a glassy

us cri

[20]

SiO2 matrix: evidence in support of the quantum-confinement mechanism Phys. Rev. B 51 1658 [22]

Philipp H, Taft E 1959 Optical constants of germanium in the region 1 to 10 eV Phys. Rev. 113 1002

[23]

Michel J, Liu J, Kimerling L C 2010 High-performance Ge-on-Si photodetectors Nat. Photonics 4 527-34

Hekmatshoar B, Shahrjerdi D, Hopstaken M, Fogel K, Sadana D K 2012 High-efficiency

an

[24]

heterojunction solar cells on crystalline germanium substrates App. Phys. Lett. 101 032102 [25]

Lv D, Gordin M L, Yi R, Xu T, Song J, Jiang Y B, Choi D, Wang D 2014 GeOx/Reduced graphene oxide composite as an anode for Li‐ion batteries: Enhanced capacity via

1059-66 [26]

Özçelik V O, Durgun E, Ciraci S 2014 New phases of germanene J. Phys. Chem. Lett. 5 2694-9

[27]

dM

reversible utilization of Li2O along with improved rate performance Adv. Funct. Mater.24

Bianco E, Butler S, Jiang S, Restrepo O D, Windl W, Goldberger J E 2013 Stability and exfoliation of germanane: a germanium graphane analogue Acs Nano 7 4414-21 Li L, Lu S z, Pan J, Qin Z, Wang Y q, Wang Y, Cao G y, Du S, Gao H J 2014 Buckled

pte

[28]

germanene formation on Pt (111) Adv. Mater. 26 4820-4 [29]

Cahangirov S, Topsakal M, Aktürk E, Şahin H, Ciraci S 2009 Two-and one-dimensional honeycomb structures of silicon and germanium Phys. Rev Lett. 102 236804

[30]

Barbagiovanni E G, Lockwood D J, Simpson P J, Goncharova L V 2012 Quantum

ce

confinement in Si and Ge nanostructures J. Appl. Phys. 111 034307

[31]

Carolan D, Doyle H 2014 Size and emission color tuning in the solution phase synthesis of highly luminescent germanium nanocrystals J. Mater. Chem. C 2 3562-8

[32]

Yuan F-W, Tuan H-Y 2014 Scalable solution-grown high-germanium-nanoparticle-

Ac

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

Page 34 of 38

loading graphene nanocomposites as high-performance lithium-ion battery electrodes: An

34

Page 35 of 38

pt

example of a graphene-based platform toward practical full-cell applications Chem. Mater. 26 2172-9 [33]

Song Z, Xu T, Gordin M L, Jiang Y-B, Bae I-T, Xiao Q, Zhan H, Liu J, Wang D 2012

us cri

Polymer–graphene nanocomposites as ultrafast-charge and-discharge cathodes for rechargeable lithium batteries Nano Lett. 12 2205-11 [34]

Yang S, Feng X, Ivanovici S, Müllen K 2010 Fabrication of graphene‐encapsulated oxide nanoparticles: towards high‐performance anode materials for lithium storage Angew. Chem. 122 8586-9

[35]

Dreyer D R, Park S, Bielawski C W, Ruoff R S 2010 The chemistry of graphene oxide Chem. Soc. Rev. 39 228-40

Yang D, et al 2009 Chemical analysis of graphene oxide films after heat and chemical

an

[36]

treatments by X-ray photoelectron and Micro-Raman spectroscopy Carbon 47 145-52 [37]

Eda G, Lin Y Y, Mattevi C, Yamaguchi H, Chen H A, Chen I, Chen C W, Chhowalla M

505-9 [38]

dM

2010 Blue photoluminescence from chemically derived graphene oxide Adv. Mater. 22

Mkhoyan K A, Contryman A W, Silcox J, Stewart D A, Eda G, Mattevi C, Miller S, Chhowalla M 2009 Atomic and electronic structure of graphene-oxide Nano Lett. 9 105863

[39]

Park S, Ruoff R S 2009 Chemical methods for the production of graphenes Nature Nanotechnol. 4 217-24

Wan L, Liu P, Zhang T, Duan Y, Zhang J 2014 Exfoliation and reduction of graphene

pte

[40]

oxide at low temperature and its resulting electrocapacitive properties J. Mater. Sci. 49 4989-97 [41]

Hummers Jr W S, Offeman R E 1958 Preparation of graphitic oxide J. Am. Chem. Soc. 80 1339

Botas C, et al 2013 Graphene materials with different structures prepared from the same

ce

[42]

graphite by the Hummers and Brodie methods Carbon 65 156-64

[43]

Pengyu L, Na L, Rongzhen S, Bing L, Xing W 2007 ICP-AES determination of germanium in GdSiGe series alloys as magnetic refrigeration material J. Rare Earth 25 377-80

Ac

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

AUTHOR SUBMITTED MANUSCRIPT - NANO-115161.R1

35

AUTHOR SUBMITTED MANUSCRIPT - NANO-115161.R1

Loryuenyong V, Totepvimarn K, Eimburanapravat P, Boonchompoo W, Buasri A 2013

pt

[44]

Preparation and characterization of reduced graphene oxide sheets via water-based

[45]

us cri

exfoliation and reduction methods Adv. Mater. Sci. Eng. 923403

Lerf A, He H, Forster M, Klinowski J 1998 Structure of graphite oxide revisited J. Phys. Chem. B 102 4477-82

[46]

Xu Y, Bai H, Lu G, Li C, Shi G 2008 Flexible graphene films via the filtration of watersoluble noncovalent functionalized graphene sheets J. Am. Chem. Soc. 130 5856-7

[47]

Chaiyakun S, Witit-Anun N, Nuntawong N, Chindaudom P, Oaew S, Kedkeaw C, Limsuwan P 2012 Preparation and characterization of graphene oxide nanosheets Procedia

[48]

Thakur S, Karak N 2012 Green reduction of graphene oxide by aqueous phytoextracts Carbon 50 5331-9

[49]

an

Engineer. 32 759-64

Zhou G, Wang D-W, Yin L-C, Li N, Li F, Cheng H-M 2012 Oxygen bridges between NiO

[50]

dM

nanosheets and graphene for improvement of lithium storage ACS Nano 6 3214-23 Zhang Y, Ma H, Zhang K, Zhang S, Wang J 2009 An improved DNA biosensor built by layer-by-layer covalent attachment of multi-walled carbon nanotubes and gold nanoparticles Electrochim. Acta 54 2385-91 [51]

Yang Z, Veinot J G 2011 Size-controlled template synthesis of metal-free germanium nanowires J. Mater. Chem. 21 16505-9

Jing C, Hou J, Xu X 2008 Fabrication and optical characteristics of thick GeO 2 sol–gel

pte

[52]

coatings Opt. Mater. 30 857-64 [53]

Pavia D L, Lampman G M, Kriz G S, Vyvyan J A 2008 Introduction to spectroscopy, Cengage Learning

[54]

Aktürk E, Ataca C, Ciraci S 2010 Effects of silicon and germanium adsorbed on graphene

ce

Appl. Phys. Lett. 96 123112

[55]

Zhou J, Song H, Ma L, Chen X 2011 Magnetite/graphene nanosheet composites: interfacial interaction and its impact on the durable high-rate performance in lithium-ion batteries Rsc Adv. 1 782-91

Ac

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

Page 36 of 38

[56]

Wepasnick K A, Smith B A, Bitter J L, Fairbrother D H 2010 Chemical and structural characterization of carbon nanotube surfaces Anal. Bioanal. Chem. 396 1003-14 36

Page 37 of 38

Yang S J, Kim T, Jung H, Park C R 2013 The effect of heating rate on porosity production

pt

[57]

during the low temperature reduction of graphite oxide Carbon 53 73-80 [58]

Sing K S 1985 Reporting physisorption data for gas/solid systems with special reference

us cri

to the determination of surface area and porosity (Recommendations 1984) Pure Appl. Chem. 57 603-19 [59]

Li D, Mueller M B, Gilje S, Kaner R B, Wallace G G 2008 Processable aqueous dispersions of graphene nanosheets Nature Nanotechnol. 3 101-5

[60]

Sun X, Liu Z, Welsher K, Robinson J T, Goodwin A, Zaric S, Dai H 2008 Nano-graphene oxide for cellular imaging and drug delivery Nano Res. 1 203-12

[61]

Taylor B R, Kauzlarich S M, Lee H W, Delgado G R 1998 Solution synthesis of germanium

[62]

an

nanocrystals demonstrating quantum confinement Chem. Mater. 10 22-4

Bahnemann D W, Kormann C, Hoffmann M R 1987 Preparation and characterization of quantum size zinc oxide: a detailed spectroscopic study J. Phys. Chem 91 3789-98

[63]

Takagahara T, Takeda K 1992 Theory of the quantum confinement effect on excitons in

[64]

dM

quantum dots of indirect-gap materials Phys. Rev. B 46 15578 Niaz S, Zdetsis A D, Badar M A, Hussain S, Sadiq I, Khan M A 2016 Optical properties of pure and mixed germanium and silicon quantum dots arXiv preprint arXiv:1602.05367 [65]

Brus L 1986 Electronic wave functions in semiconductor clusters: experiment and theory J. Phys. Chem. 90 2555-60

[66]

Samavati A, Othaman Z, Ghoshal S, Dousti M 2014 Optical and structural investigations

[67]

pte

of self-assembled Ge/Si bi-layer containing Ge QDs J. Lumin. 154 51-7 Bagani K, Bhattacharya A, Kaur J, Rai Chowdhury A, Ghosh B, Sardar M, Banerjee S 2014 Anomalous behaviour of magnetic coercivity in graphene oxide and reduced graphene oxide J. Appl. Phys. 115 023902 [68]

Kumazaki H, Hirashima D S 2007 Nonmagnetic-defect-induced magnetism in graphene J.

ce

Phys. Soc. Jpn. 76 064713-

[69]

Tang T, Tang N, Zheng Y, Wan X, Liu Y, Liu F, Xu Q, Du Y 2015 Robust magnetic moments on the basal plane of the graphene sheet effectively induced by OH groups Sci. Rep. 5 8448

Ac

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

AUTHOR SUBMITTED MANUSCRIPT - NANO-115161.R1

[70]

Wang M, Huang W, Chan-Park M B, Li C M 2011 Magnetism in oxidized graphenes with hydroxyl groups Nanotechnology 22 105702 37

AUTHOR SUBMITTED MANUSCRIPT - NANO-115161.R1

Bagani K, Ray M K, Satpati B, Ray N R, Sardar M, Banerjee S 2014 Contrasting magnetic

pt

[71]

properties of thermally and chemically reduced graphene oxide J. Phys. Chem. C 118 13254-9

Nair R, et al 2013 Dual origin of defect magnetism in graphene and its reversible switching

us cri

[72]

by molecular doping arXiv preprint arXiv:1301.7611 [73]

El-Dek S 2010 Effect of annealing temperature on the magnetic properties of CoFe2O4 nanoparticles Philos. Mag. Lett. 90 233-40

[74]

Nair R, Sepioni M, Tsai I-L, Lehtinen O, Keinonen J, Krasheninnikov A, Thomson T, Geim A, Grigorieva I 2012 Spin-half paramagnetism in graphene induced by point defects Nat. Phys. 8 199-202

Özçelik V O, Kecik D, Durgun E, Ciraci S 2014 Adsorption of group IV elements on

an

[75]

graphene, silicene, germanene, and stanene: dumbbell formation J. Phys. Chem. C 119 845-53 [76]

Ruderman M A, Kittel C 1954 Indirect exchange coupling of nuclear magnetic moments

ce

pte

dM

by conduction electrons Phys. Rev. 96 99

Ac

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

Page 38 of 38

38