Large, Linear, and Tunable Positive Magnetoresistance of

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Large, Linear, and Tunable Positive Magnetoresistance of Mechanically Stable Graphene Foam−Toward High-Performance Magnetic Field Sensors Rizwan Ur Rehman Sagar,†,‡ Massimiliano Galluzzi,†,‡ Caihua Wan,§ Khurram Shehzad,∥ Sachin T. Navale,†,‡ Tauseef Anwar,⊥ Rajaram S. Mane,#,○ Hong-Guang Piao,∇ Abid Ali,◆ and Florian J. Stadler*,† †

College of Materials Science and Engineering, Shenzhen Key Laboratory of Polymer Science and Technology, Guangdong Research Center for Interfacial Engineering of Functional Materials, Nanshan District Key Lab for Biopolymers and Safety Evaluation and ‡Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province, College of Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, PR China § Institute of Physics, Chinese Academy of Sciences, Beijing, 100190, PR China ∥ Department of Information Science and Electronic Engineering, Zhejiang University, Hangzhou 310027, PR China ⊥ Beijing Key Laboratory of Fine Ceramics, Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 100084, PR China # School of Physical Sciences, Swami Ramanand Teerth Marathwada University, Nanded 431606, India ∇ College of Science, China Three Gorges University, Yichang 443002, PR China ○ Department of Chemistry, College of Science, Bld-5, King Saud University, Riyadh, Saudi Arabia ◆ Department of Chemistry, Quaid-i-Azam University, Islamabad 45320, Pakistan S Supporting Information *

ABSTRACT: Here, we present the first observation of magneto-transport properties of graphene foam (GF) composed of a few layers in a wide temperature range of 2−300 K. Large room-temperature linear positive magnetoresistance (PMR ≈ 171% at B ≈ 9 T) has been detected. The largest PMR (∼213%) has been achieved at 2 K under a magnetic field of 9 T, which can be tuned by the addition of poly(methyl methacrylate) to the porous structure of the foam. This remarkable magnetoresistance may be the result of quadratic magnetoresistance. The excellent magneto-transport properties of GF open a way toward threedimensional graphene-based magnetoelectronic devices. KEYWORDS: graphene foam, chemical vaport deposition and magnetoresistance

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hierarchical architectures.8 Construction of 3D hierarchical architectures composed of 2D materials can be an important task, which would be a central part in commercializing their applications, as these architectures provide high specific surface areas, good mechanical properties, and fast mass and electron transport characteristics due to combination of 3D porous architectures of 2D materials with excellent intrinsic properties of 2D materials such as graphene foam (GF). 3D/bulk morphology of graphene (i.e., GF, etc.) is proffered in batteries,9,10 supercapacitors,11 and flexible electronic devices etc.12 Moreover, gram-scale production of 2D graphene is a challenging task, due to the large surface to volume ratio of monolayer graphene, while 3D GF can be produced in gram-

INTRODUCTION Unique electro/magneto-transport properties arising from the flat two-dimensional (2D) structure of graphene can be tuned by imparting structural changes, such as reducing size (i.e., nanoribbons strategy1 and quantum dots2), chemical functionalization,3 or changing the number of layers, among others.4,5 For example, single-layered graphene is considered as a zero band gap material,6 while the band gap of bilayered graphene (250 meV) opens under electric field,4 and trilayer/few-layers of graphene can have a tunable band gap depending upon the stacking.5 In addition to structure, morphology of graphene also plays an important role in determining their electro/magneto transport properties. For example, Yue et al.7 fabricated petal-/ tree-like graphene architecture with positive or negative magnetoresistance (MR) behavior depending upon the morphology. 2D materials used in energy, environment, sensing, and biological fields often require assembly of well-defined 3D/bulk © 2016 American Chemical Society

Received: October 13, 2016 Accepted: December 15, 2016 Published: December 15, 2016 1891

DOI: 10.1021/acsami.6b13044 ACS Appl. Mater. Interfaces 2017, 9, 1891−1898

Research Article

ACS Applied Materials & Interfaces

Figure 1. (a) HRTEM of FLGF (inset is corresponding selected area diffraction pattern), (b) Raman spectrum of FLGF, (c) surface via AFM, (d) step profile of FLFG, and (e) Multi-Gaussian quantitative analysis (used for confirming FLG) of FLGF surface.

scale easily.9 Large-area single-crystal graphene is not realized to the date, which is the biggest hurdle for the utilization of graphene in practical electronic gadgets.13 According to the roadmap of Novoselov et al.,8 high-quality large-area graphene sheets will be available in 2030−2035, indicating unavailability of large-area single-crystal graphene at present. Thus, graphene research field requires hierarchy of 3D structure of 2D graphene such as GF for graphene-based practical devices.14 To the best of our knowledge, magneto-transport properties of bulk GF have not been explored so far. Recently, Peng et al.14 reported positive magnetoresistance (PMR ≈ 180% at 300 K) of multilayer graphene after breaking GF. Magnetotransport properties of GF could be different in comparison to those of graphene sheets due to the presence of different sizes of graphene flakes and more importantly types of connection between these flakes, presence of edge boundaries,15 geometry of graphene edges,16 difference in the stacking of graphene layers, and different number of graphene layers.4,5,17 Therefore, graphene morphology has influence on the motion and trajectories of the charge carriers under the applied magnetic field; thus, interesting electro/magneto transport properties can be observed. However, metal electrode deposition on the top of GF is a big challenge for studying electro/magneto transport properties.18 Herein, for the first time, our group has employed a novel method to measure MR of GF by designing an electrical unit with six-probe system and measured the electric/magneto transport properties. These probes were prepared by using nonmagnetic materials like gold and copper. Moreover, a fourprobe unit is favorable, owing to its basic advantage that the resistance of the electrodes is excluded during the measurement. The extension to a six-probe unit is an augmentation of this construction for measurements of Hall mobility. The

magneto-transport properties of GF have been measured from 2 to 300 K under the application of maximum magnetic field of 9 T, which has not been achieved previously to the best of our knowledge. We detected large and linear positive magnetoresistance (LPMR) under the application of a magnetic field.

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EXPERIMENTAL METHODS

Growth Conditions. The GF was synthesized by using the chemical vapor deposition (CVD-GSL-1700X, MTI) method as described in the literature (Figure S1).9,11,39 Nickel foam was used as substrate for this purpose, and a mixture of three gases was utilized. These gases were inserted from one end of the furnace tube and sucked out from other end via a rotary vacuum pump (Figure S1a). Hydrogen and argon gases were provided from the beginning of the experiment until the end at the flow rate of 40 standard centimeter cube per minute (sccm) and 160 sccm, respectively (Figure S1b). Ethylene (C2H4) gas was used as carbon precursor and provided at elevated temperature of 1050 °C for 2 to 10 min, depending upon the required number of graphene layers. The C2H4 gas stopped mixing after particular growth time; then, specimens were taken out of the furnace upon reaching at room temperature. Transfer of GF. As-grown graphene on nickel was dipped into the solution of FeCl3 for three days. The specimen sinks to the bottom of the solution due to the presence of nickel. GF starts floating on the surface of the FeCl3 solution as soon as nickel was etched away. GF was removed from the solution and washed with acetone several times. The resulting GF was immersed in acetone for several days to prevent any contamination, if there was any. Characterization of GF. After transferring process, structure of GF was accessed via high-resolution transmission electron microscope (HRTEM, JEOL-2010) and Raman spectroscopy (Renishaw-HR800). Atomic force microscopy (AFM-Dimension, Bruker) image was utilized to confirm thickness of GF. Scanning electron microscopy (SEM-Hitachi, SU-70) was used for surface and morphology analyses (Figure S2). X-ray photoelectron spectroscopy (XPS, Escalab-250Xi, Thermo Scientific, USA) was used for the observation of the chemical 1892

DOI: 10.1021/acsami.6b13044 ACS Appl. Mater. Interfaces 2017, 9, 1891−1898

Research Article

ACS Applied Materials & Interfaces state of surface atoms. The magneto-transport properties were measured by using a Physical Property Measurement System (PPMS-9T, Quantum design, Inc. USA) in the temperature range from 300 to 2 K. Electric/Magneto Transport Measurement Unit. Metal electrodes on the top/bottom of a single layer of graphene play a key role to detect accurate electric transport properties. In general, metal electrodes are deposited on top of graphene layer, as well as pattern graphene to avoid fringing field effect, which is the standard procedure for thin material. However, GF is considered as a bulk specimen due to thickness of several millimeters and macroscopic measurement was done. Thus, no metal electrodes were deposited at the top of GF. The electric/magneto transport properties of GF were studied with the help of a six-probe unit, which was fabricated, as shown in Figure S2. Each probe was made of gold-coated copper. Current was flowing between two electrodes (channel length: 10 mm), while the voltage was detected across other two electrodes (channel length: 4 mm and diameter: 500 μm; for more details, see Figure S2). The diameter of the electrode was ∼500 μm, which is a typical thickness for macroscopic measurements. Composite of GF. Poly(methyl methacrylate) (PMMA-SigmaAldrich, CAS# 9011−14−7) was dissolved in acetone with different PMMA concentrations. Two solutions of PMMA/acetone were prepared.

suggesting the presence of few-layer graphene. The influence of the defects on the vibrational response of GF can be observed via D and G bands. Thus, crystallite size ∼67 nm is calculated from ID/IG intensity ratio according to literature.27 The crystallite size measured via Raman depends upon intensity and area under the D and G band. Voiry et al.27 reported the crystallite size of high-quality CVD-graphene as ∼186 nm and that of dispersed graphene as ∼90.9 nm. Thus, GF crystallite size of ∼67 nm is lower as compared to those of CVDgraphene and dispersed graphene owing to the difference of morphology. The thickness of the material is sensitive to the number of graphene layers in FLGF, which was determined by using AFM (Figure 1c−e). A monolayer of graphene is atomically thin, suggesting its thickness must be less than 1 nm and thus can be classified as a perfect 2D material. The thickness of FLFG has been estimated via AFM operating in PeakForce Tapping mode, as each layer adds up thickness. Quantitative analysis by multi-Gaussian fitting was used to estimate the thickness of layers (i.e., impurities or defects were neglected). The thickness ranges from 3.71 ± 0.12 nm to 4.82 ± 0.14 nm (Figure 1c−e). Magneto-transport properties of GF measured for different temperatures in the range between 2 to 300 K with the change of the magnetic field from 0 to 9 T. The resistance of the specimen is decreased with an increase of the temperature from 2 to 300 K (Figure S6). The MR was measured by the following relation28−30

1. 15% of PMMA + 85% acetone 2. 30% of PMMA + 70% acetone As a first step, GF was placed in a box-shaped die and then PMMA solution was poured from the top. This solution was absorbed from top to bottom and specimens were dried at 50 °C overnight in heating oven (Shanghai HongHua, DHG-9070A).

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⎛ R (B ) ⎞ MR (%) = ⎜ xx − 1⎟ × 100% ⎝ R xx(0) ⎠

RESULTS AND DISCUSSION Few-layered graphene foam (FLGF) was prepared via CVD method in accordance with previous literature.9,11 FLGF was transferred from the nickel foam on which it was grown for future characterization.9 The morphology of GF consists of randomly connected graphene sheets (Figure S3a−f). The structure of GF was successfully observed by HRTEM (Figure 1a). A spacing of ∼0.34 nm has been detected between each atomic plane, corresponding to graphene layer spacing. Selected area electron diffraction pattern reveals a 6-fold symmetric diffraction pattern (inset of Figure 1a), matching well with literature and being similar to the structure of graphene.19,20 Moreover, XPS and energy dispersive X-ray (EDX) spectra did not show any traces of iron and nickel impurities in graphene foam (Figures S4 and S5), suggesting phase purity of obtained FLGF. XPS survey was conducted for three different specimens for the confirmation of the two peaks (∼ 950 and 1250 eV), which were repeatedly found in all three specimens, indicating that these peaks are not from GF. FLGF shows three prominent peaks in the Raman scattering spectrum; D, G, and 2D (Figure 1b). The D peak is the result of breathing mode of the six ring atoms present in graphene and caused by the presence of defects in graphene,21 the presence of high-frequency E2g phonons generates the G peak, and the 2D peak is the overtone of the D peak, which is always present in graphene and has no relation with defects.22,23 The D, G, and 2D peaks, in accordance with literature values, are found at ∼1330, ∼1580, and ∼2700 cm−1, respectively.24 The ratio of the intensities of G (IG) and 2D (I2D) peaks hint about the number of graphene layers, as the Raman mechanism is closely linked to the electronic structure of the material.25 Intensity ratio I2D/IG > 1 represents single-layer graphene,26 while I2D/IG < 1 represents few-layer graphene. In our case, the intensity ratio of the FLG is I2D/IG ≈ 0.98 (Figure 1b),

(1)

where, Rxx(B) is the resistance at magnetic field B and Rxx(0) is the resistance at zero magnetic field. The profile of the resistance versus magnetic field (R−B) curve changes from cusp to parabolic shape at 2−300 K, respectively (Figure 2a). It is corroborated that the resistance of the specimen is increased with the increase of magnetic field (Figure S7a). The resistance of the specimens under lower magnetic fields presents a linear trend, while the linear profile of the temperature-dependent resistance curve is abruptly shifted at vicinity of 50 K under higher magnetic fields (Figure S7a). PMR versus magnetic field (PMRB) curve has changed its shape from cusp to parabolic from 2 to 300 K, respectively (Figure 2b). A linear trend of MR versus temperature (PMRT) has been observed under different magnetic fields (Figure S7b). The highest obtained magnitude of PMR ≈ 213% is detected at 2 K under 9 T. A detailed comparison of literature reports on PMR in graphene-based materials and FLGF is shown in Table. 1 (i.e., estimated via eq 1 at 300 K and 5 T). A PMR of 30% was estimated in graphite, which is increased to 75 (±5) % at 2 K.31 A very low magnitude of PMR has been detected in petal-/treelike graphene.7 Graphene oxide (GO), also a graphene-based material, shows 50% PMR at 300 K.32 Recently, Peng et al.14 observed exciting results from multilayer graphene obtained from the breaking of 3D porous structure of GF, wherein highest magnitude of MR in multilayer graphene has been reported to date. The PMR of few layers of graphene (FLG) at 300 K and 5 T is higher as compared to graphite.33 Bilayer graphene opens a band gap under the applied electric field that is considered as semiconducting material.4 Moreover, band gap of trilayer graphene also opens band gap under the applied electric field.5 The largest magnitude of PMR is confirmed in 1893

DOI: 10.1021/acsami.6b13044 ACS Appl. Mater. Interfaces 2017, 9, 1891−1898

Research Article

ACS Applied Materials & Interfaces

observed in GF like morphology. The morphology of FLGF is not comparable to bilayer or monolayer sheetlike graphene, as monolayer and bilayer graphene are purely flat. Moreover, morphology of GF cannot be compared with graphite, multilayer graphene, and few-layer graphene. However, magnetotransport properties of GF are better in comparison to graphite, petal-/tree-like graphene and GO (Table. 1). FLGF is a combination of graphene layers of different sizes and orientations, creating multiple conduction regimes (i.e., due to presence of randomly oriented few layers graphene in GF). Therefore, interfaces, boundaries, and defects significantly influence electro/magneto transport properties. Thus, classical interpretation of PMR in FLGF will be considered first and it is termed as quadratic MR:34

MR ∝ (μB)2

where μ is the mobility and B is applied magnetic field, termed as quadratic MR coefficient. Lorentz force curves the path of carriers, which results into a higher resistance under the application of a magnetic field. As a result, PMR is detected in FLGF. MR (%) versus B2 curve shows a nonlinear trend at low magnetic field, while it is linear at medium and high fields (Figure 3a). PMRB curve has been fitted with the help of eq 2 at low magnetic field regime and mobility was estimated from the fitting (Figures 3b and S8). The mobility decreased with the increase of temperature from 2 to 300 K, which is similar to MR results. The experimental mobility can be estimated from Hall resistance versus magnetic field curve (Figure S9), which requires accurate thickness of the specimen. Hall coefficient (RH) can be calculated by multiplying the slope of the Hall resistance versus magnetic field curve with the thickness of the specimen. Mobility (μ) can only be estimated if RH is measured accurately as μ = σRH, and that is only possible if the precise thickness of the specimen is known. As GF has many pores and surface is not flat, the exact geometry of the specimen cannot be determined satisfactorily. In addition, two types of quantum scatterings are responsible during quantum transport in graphene: (i) inelastic (phase breaking, τϕ) and (ii) elastic (chirality breaking, τi, τw).35

Figure 2. (a) Resistance vs magnetic field and (b) MR vs magnetic field measurements of FLGF at different temperatures.

Table 1. Comparison of PMR in Graphene-Based Materialsa graphene-based materials graphite petal-/tree-like graphene graphene oxide multilayer graphene sheets multilayer graphene sheets from GF few layers of graphene sheets bilayer graphene sheet monolayer graphene sheet few layers of graphene foam (FLGF) a

PMR (%) at 300 K and 5 T

PMR (%) at 2 K and 5 T

ref.

30

70−80

31