Green reduction and patterning of graphene oxide via photothermal and electrochemical methods

DECLARATION I hereby declare that the thesis is my original work and it has been written by me in its entirety. I have duly acknowledged all the sources of information which have been used in the thesis. This thesis has also not been submitted for any degree in any university previously. ____________ Tao Ye 21st May 2013 i Acknowledgements It has been an exciting and fulfilling experience for me to have spent the last three years in the Nanomaterials Research Lab at the National University of Singapore. Special thanks had to be given to my supervisor, Associated Professor Sow ChorngHaur for his contagious passion and optimism that encourages me on, valuable ideas to direct me through the bottle-necks in research, and constant help and guidance in the everyday experiments. No matter where I would be in the future, I will be sure to take along with me this enriching and unforgettable experience. Hearfelt gratitude must be given to Dr. Binni Varghese and Ms Sharon Lim for interesting suggestions and numerous help, and to Mr. Teoh Hao Fatt for all the collaborations. I would also thank Prof Tok Eng Soon, Dr Zhang Zheng for helping with XPS measurements; Dr. Wang Shuai and Prof Loh Kian Ping for supplying with experimental materials; Dr. Cong Chunxiao and Asst Prof Yu Ting for micro Raman mapping. Sincere appreciation must also be given to my fellow labmates Mr. Zheng Minrui, Mr. Lu Junpeng, Mr. Xi Yilin, Mr. Chang Sheh Lit, Mr. Rajesh, Mr. Rajiv, Ms. Loh Pui Yee, Mr. Lee Kian Keat, and all the technicians Ms. Foo Eng Tin, Mr. Chen Gen Seng, Mr. Ong for all the help I had received over the time and for making the lab a warm and homely place to work in. Last but not least, I would like to thank my family and friends for their endless support and for always being there for me through all difficulties and frustrations. I hereby dedicate this piece of work to them. ii Table of Contents Acknowledgements ..................................................................................................... ii Table of Contents ...................................................................................................... iii Abstract ...................................................................................................................... vi List of Publications and Presentations ................................................................... vii List of Figures .......................................................................................................... viii Chapter 1: Introduction ............................................................................................ 1 References ................................................................................................................ 3 Chapter 2: Theoretical Background ........................................................................ 4 2.1 Graphene ...................................................................................................... 4 2.1.1 Structure and Properties ........................................................................... 4 2.1.2 Synthesis and Modification ...................................................................... 7 2.2 Graphene Oxide ........................................................................................... 8 2.2.1 Structure ................................................................................................... 9 2.2.2 Synthesis ................................................................................................ 10 2.2.3 Properties ............................................................................................... 12 2.3 Reduced Graphene Oxide .......................................................................... 14 2.3.1 Structure ................................................................................................. 14 2.3.2 Reduction of Graphene Oxide................................................................ 15 2.3.3 Properties ............................................................................................... 17 2.4 Applications of Graphene Oxide-based Materials ..................................... 21 2.4.1 Thin Films of GO or RGO ..................................................................... 21 2.4.2 Transparent Conductor ........................................................................... 21 2.4.3 Sensing ................................................................................................... 22 2.4.4 Precursor to Graphitic Nanostructure .................................................... 22 2.4.5 Precursor to Graphene-based Composites ............................................. 22 References .............................................................................................................. 24 Chapter 3: Experimental Methods ......................................................................... 29 3.1 Sample Preparation .................................................................................... 29 3.2 Direct Writing with Focused Laser System ............................................... 31 3.3 Electrical Measurement .............................................................................. 34 3.4 Raman Spectroscopy .................................................................................. 35 iii 3.4.1 Basic Principles ...................................................................................... 35 3.4.2 Vibrational States ................................................................................... 37 3.4.3 Raman Spectrum of Graphite-based Materials ...................................... 38 3.4.4 Raman Spectra for Defective Graphite .................................................. 41 3.4.5 Substrate Effect ...................................................................................... 42 3.5 X-ray Photoemission Spectroscopy ........................................................... 43 3.6 AFM ........................................................................................................... 43 3.7 Microscopy and Spectrometer.................................................................... 44 References .............................................................................................................. 45 Chapter 4: Photothermal Reduction ...................................................................... 47 4.1 Introduction ................................................................................................ 47 4.2 Conductivity change................................................................................... 48 4.2.1 Increased Electrical Conductivity .......................................................... 49 4.2.2 Contact Resistance ................................................................................. 51 4.2.3 Effect of Channel Dimensions ............................................................... 52 4.2.4 Effect of Repeated Irradiation ................................................................ 53 4.3 Chemical Composition ............................................................................... 56 4.3.1 Raman Spectroscopy .............................................................................. 56 4.3.2 XPS ........................................................................................................ 58 4.4 Morphological Change ............................................................................... 60 4.4.1 Film Thickness ....................................................................................... 60 4.4.2 Optical Contrast ..................................................................................... 68 4.4.3 Patterning Ability ................................................................................... 72 4.5 Surface Properties ...................................................................................... 73 4.6 Proposed Mechanism of Reduction ........................................................... 74 4.7 Conclusion.................................................................................................. 75 References .............................................................................................................. 76 Chapter 5: Visible Electrochemical Reduction ..................................................... 78 5.1 Introduction ................................................................................................ 78 5.2 Electrochemical Reduction ........................................................................ 79 5.2.1 Directional .............................................................................................. 79 5.2.2 Reversibility ........................................................................................... 80 5.3 Change of Properties .................................................................................. 82 iv 5.3.1 Chemical Composition ........................................................................... 82 5.3.2 Morphological Change ........................................................................... 84 5.4 Temporal Behavior..................................................................................... 86 5.4.1 Area Change with respect to Time ......................................................... 86 5.4.2 Area Change correlated to Conductivity ................................................ 90 5.5 Mechanism ................................................................................................. 93 5.5.1 5.6 Moist assisted Redox Reaction .............................................................. 93 Conclusion.................................................................................................. 96 References .............................................................................................................. 97 Chapter 6: Conclusion ............................................................................................. 98 v Abstract The exfoliation of graphite oxide into graphene oxide (GO), and its subsequent reduction to reduced graphene oxide (RGO), came into research attention initially as a chemical synthesis route for large-quantity of solution-processable graphene; and later for the unique properties and potential of graphene oxide (GO) and reduced graphene oxide (RGO) themselves in applications, such as transparent conductors and super capacitors. Most common methods of reduction to synthesize RGO require hazardous chemicals as the reduction agents, which introduces additional contaminations. However, here we investigated and compared two different approaches for the green synthesis of RGO in ambient environment. We first employed a focused laser beam system to locally reduce GO thin film deposited on silicon-based substrates. The changes in the morphology, chemical composition and electrical properties were studied between GO and RGO to reveal the mechanism of the reduction via photothermal removal of the oxygen-containing functional groups. The RGO had a high concentration of residual defects, similar to other works. Therefore it could not demonstrate Quantum Hall Effect or ballistic transport as graphene. However, a significant increase in its conductivity was observed upon reduction. Moreover, controlled and facile patterning of the sample could be achieved to produce continuous 2-dimensional carbon matrix with different electrical properties. More complicated 3-dimensional structuring could also be achieved. We then also investigated the reduction of GO via direct current. The reduction process was reversible and optically observable in real-time. The properties of electrically reduced GO (ERG) were also studied to confirm the change in its morphology, chemical composition and electrical properties were similar to the product of photothermal reduction. The process was tracked to elucidate the mechanism of the electrochemical reduction. Finally the two different synthesis route was combined for guided-electrochemical reduction of GO. vi List of Publications and Presentations 1. Ye Tao, Binni Varghese, Manu Jaiswal, Shuai Wang, Zheng Zhang, Barbaros Oezyilmaz, Kian Ping Loh, Eng Soon Tok, Chorng Haur Sow Localized insulator-conductor transformation of graphene oxide thin films via focused laser beam irradiation Appl Phys A 106, 523-531 (2012) 2. Hao Fatt Teoh, Ye Tao, Eng Soon Tok, Ghim Wei Ho, Chorng Haur Sow Direct laserenabled graphene oxide–Reduced graphene oxide layered structures with micropatterning J. Appl. Phys. 112, 064309 (2012) 3. H. F. Teoh, Y. Tao, E. S. Tok, G. W. Ho, and C. H. Sow Electrical current mediated interconversion between graphene oxide to reduced grapene oxide Applied Physics Letters 98, 1 (2011) 4. Conference presentation: Recent Advances in Graphene and Related Materials Localized Insulator-Conductor Transformation of Graphene Oxide Film via Focused Laser Beam Irradiation (2010) vii List of Figures Figure 3.1A) Schematics of the Laser Beam System to focus laser onto sample placed on the computer controlled x-y stage; B) Schematic diagram depicting focused laser beam following onto the GO film on substrate with pre-deposited gold electrodes for electrical measurement. ....................................................... 33 Figure 3.2 Basic principle of Raman Scattering ........................................................ 36 Figure 4.1 A) Optical micrograph of GO film after pruning a single channel across the gold electrode (insert is the optical image of the as-deposited GO film); B) IV curves recorded from the as-deposited (black line) and laser pruned (blue line) GO film. ..................................................................................................... 48 Figure 4.2 Mobility measurement from the single channel ....................................... 50 Figure 4.3 Comparison of I-V curve between different laser irradiated patterns: A) single channel scanned (left optical image in the insert) and large area scanned at contacts (right optical image in the insert); B) Single channel with contacts scanned (left optical image in the insert) and additional large area scanned in the centre (right optical image in the insert). ........................................................... 52 Figure 4.4 A) i) to v) Optical microscopy image of the channel with increasing width vi) Schematic representation of the laser-scanning sequence to create a conducting channel with fixed length and increasing width; B) I-V curves recorded from channels i) to v); C) A plot of conductance of laser created channel as a function of channel width .............................................................. 53 Figure 4.5 A) Optical micrograph to show the repeatedly irradiated area between the electrodes; B) Change of conductance of reduced GO with repeated laser irradiation, each time with 6 mW irradiation power. ......................................... 54 Figure 4.6 A) Raman spectrum and B) Raman-mapping of as-deposited and laserpruned GO .......................................................................................................... 57 Figure 4.7 A) C1s scan of XPS for GO and rGO; B) [pending] O1s scan for GO and rGO..................................................................................................................... 58 Figure 4.8 XPS Spectrum of A) as-deposited and B) laser-irradiated GO film for valence band. ...................................................................................................... 59 Figure 4.9 Change in film thickness due to laser irradiation as measured by AFM. A) AFM image of single laser irradiated channel across two electrodes B) AFM scan of the area enclosed by dotted line in Figure A and its corresponding height profile; C) AFM image of a scratch on the GO film; D) Height profile near the scratched region (blue) and fitted difference in film thickness (red) ................. 61 viii Figure 4.10 A) AFM image of the laser irradiated channel and a nearby scratch; B) height profile along the line indicated in the left image (blue) and fitted change in film thickness (red). The film thickness is ~16nm and sunken depth is ~6nm. ............................................................................................................................ 63 Figure 4.11 A) AFM image of the laser irradiated area, including the electrodes; B) AFM image of the area enclosed by dotted line in Image (A), GO film to the left of the dotted line was laser-irradiated, to the right was as-deposited; C) AFM image of a nearby scratch; D) Height profile along the line indicated in image C (blue) and fitted difference in film thickness (red). ........................................... 64 Figure 4.12 The plot of the remaining thickness of laser-irradiated region against the original film thickenss. Laser of 532nm, 10mW was focused over a 1μm by 2μm region and scanned over the sample at 10μm/s. ....................................... 65 Figure 4.13 AFM images of 8 line cuts with 1 to 8 times of laser irradiation each (above) and the height profile along the line drawn in AFM image (below), indicating no significant difference between sunken depth for four different sample with sunken depth of A)300nm in wavelength, accounted by increases in the extent of graphitization. 2.4.3 Sensing GO can also exhibit non-covalent binding on the sp2 networks that are not oxidized or engaged in hydrogen bonding. Lu et al. reported a DNA sensor that utilized a noncovalent binding interaction between DNA or proteins and GO platelets [109] demonstrating that this material holds promise as a platform for sensitive and selective detection of DNA and proteins. The authors suggested π-π interactions and hydrophobic interactions between GO and doxorubicin hydrochloride (DXR) were the primary interactions that linked the two units together. 2.4.4 Precursor to Graphitic Nanostructure The presence of defects due to oxygen groups creates chemically reactive sites that allow GO to be cleaved into smaller sheets [110] by chemical or physical means, generating nanosized GO or nanoribbons that have markedly different properties from the micrometer-sized counterpart. GO can be disintegrated into small fragments and poly-aromatics by sonochemical treatment in acids [111], which can be reconstituted into fullerenes and carbon wires. Other methods of fabrication of nanosized GO include hydrothermal cleave of GO to graphene quantum dots in suspension [112]. 2.4.5 Precursor to Graphene-based Composites GO can be utilized as a minor filler component embedded within either a polymer or an inorganic matrix due to the rich oxygen content on its surface. A variety of GObased nanocomposites have been prepared as thin films, and commonly reduced to RGO composites for conductivity studies. Spin-cast films of silica containing up to 11wt% of GO have been prepared and reduced to give transparent conductive layers [113]. Paperlike thin films with up to 1.4 wt% of polystyrene have been made by co- 22 filtration of an aqueous solution containing both GO and polystyrene nanoparticles [87]. Drop-cast polyurethane films containing 4.4 wt% of GO were prepared and found to increased Young’s modulus by an impressive ~900% in comparison to films of the pristine polymer with only minimal decrease in tensile strength [114]. Chemical or thermal reduction of GO, on the other hand, partially restore the graphitic network in the basal plane of RGO, as was discussed previously. 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X. Xu et al., J. Am. Chem. Soc. 130, 5856 (2008). [117] J. R. Lomeda et al., J. Am. Chem. Soc. 130, 16201 (2008). [118] M. S. Dresselhaus, and G. Dresselhaus, Adv. Phys. 30, 139 (1981). [119] M. Winter et al., Advanced Materials 10, 725 (1998). 28 Chapter 3: Experimental Methods 3.1 Sample Preparation As explained in Section 1.2, suspension of graphene oxide (GO) in various solvents can be readily synthesized via oxidation of commercial grade graphite. For application and characterization, it is necessary to fabricate a uniform, single graphene oxide film from the suspensions. Such efforts can be traced back before the discovery of graphene, to works on conductive composite films of alternating GO and polyelectrolyte layers [1]. Thin films with spotty coverage were prepared by drop casting GO suspensions on a silica substrate or briefly immersing the substrate within the suspensions. While films with near monolayer thickness of GO were claimed, coverage >60% was only attained in multilayered structures [1-3]. Such films were found to be effective hole conductors but unstable and readily reduced during electrochemical analysis. Recently spin-coating provides an easy means for producing continuous films composed of GO sheets. Spin-coating involves depositing a small puddle of excess fluid onto the centre of a substrate and then spinning the substrate at high speed. Centrifugal force will spread the fluid to and eventually off the edge of the substrate, forming a thin film on the surface. The film thickness depends on nature of the fluid and substrate such as viscosity, rate of evaporation, percent of suspension, surface tension; as well as experimental parameters such as angular velocity and acceleration. From earlier works, the thickness of deposited layer can be tuned by varying the concentration of the GO dispersion with 2mg mL-1 giving ~3nm thick films and higher concentrations 12-15 mg mL-1 yielding films ~20nm thick [4]. Most spin-coated films are unlikely true monolayer films as they are thicker than individual GO sheets. Patterning of GO sheets on the surface of a substrate can be achieved via additional step of templating [5], etching [6]or direct-writing [7]. The fabrication of monolayer GO thin film with good coverage was also reported using Langmuir-Blodgett assembly techniques from basic dispersions of GO [8]. The formation of multilayer structures was discouraged due to the repulsive negative charges from deprotonated carboxylic acid groups along the edges of adjacent GO 29 sheets. At high compression, the folded edge layers would partially overlap and interlock to create a continuous film with a relatively uniform monolayer thickness. Monolayer was ineffective in our experiments and this technique was therefore not used. To fabricate the GO thin film used in our experiments, 1.5 g of graphite, 1.5 g of NaNO3 and 69 ml of H2SO4 were first mixed and stirred in an ice bath. Next, 9 g of KMnO4 was slowly added. The solution was kept in room temperature and stirred for 1 h. After which, 100 ml of water was added and the temperature was increased to 90 °C. After 30 mins, 300 ml of water followed by 10 ml of H2O2 were slowly added. The resultant mixture was filtered and washed by water until the solution pH was about 6. The synthesized GO sheets were dispersed in water: methanol (1:5) mixture and purified with 3 repeated centrifugation steps at 12,000 rpm for 30 mins. The purified sample was centrifuged at 8,000 rpm for 30 min to remove the smaller sized GO sheets. Then the remaining solution was redispersed in water/methanol mixture with the ratio of 1:5 and centrifuged at 2,500 rpm to recover the GO sheets. The GO sheet aqueous solutions were made by centrifugation of the obtained GO solution and followed by re-dispersion into water. 1 mg/ml, 4 mg/ml and 6mg/ml GO aqueous solution was drop-cast or spin coated onto SiO2/Si substrate unless otherwise stated. Various thin films with different thicknesses ranging from a few nm to ~1μm were prepared. As an example, spin-coating with a 6mg/ml GO solution on a SiO2/Si substrate was carried out at 800rpm for 5 minutes to give a film 30~50 nm in thickness for electrochemical studies. All thin films used were thicker than a few nanometers and therefore not monolayers. Similar to previous works from literature, monolayer could be deposited by high speed spin-coating, but deposited thin films would have poor coverage and therefore would not be suitable for investigation with electrical methods. 30 3.2 Direct Writing with Focused Laser System In a wide variety of applications, it is desirable to pattern conductive thin films at precise positions for connection with other components, or simply for the control of its electrical property. The patterning of graphene oxide thin film prior to or after reduction has therefore been a key interest of research in fabrication of graphenebased devices. Existing methods of patterning graphene include mask lithography, transfer printing, and direct-writing process. The use of optical or shadow masks for lithography or etching is a common method used in semiconductor fabrication industry. It is therefore readily up-scalabe for large scale preparation. In the case of graphene, oxygen reactive ion etching (RIE) is generally used to remove material not covered by the mask. Examples of masks used include self-assembly of polystyrene nanosphere to produce periodically ordered graphene nanodisk arrays [9], or aluminium evaporated over a copper shadow mask to fabricate electrodes for organic field emission transistors [10]. Although suitable for large-scale patterning, this method is also limited by under-etching and contamination from the contacting masks and washing solvents. Transfer-print of graphene layers onto different types of substrates is also used for a wide range of applications as a soft contact method well-suited for the generation of graphene pattern. Graphene on glass substrates was contacted with a patterned poly(dimethylsiloxane) (PDMS) stamp[11]. The “inked” stamp would contact with a Si/SiO2 substrate again. As the binding energy of PDMS for graphene is weaker than the substrate interface, graphene is readily transferred from PDMS to Si/SiO2 substrates at room temperature. PDMS stamp-based cutting and exfoliation was used for production of polymer/graphene hybrid films [12], or for precise transfer-print of graphene patterns in device-active areas [13]. The exfoliation process can be enhanced by applying a voltage between graphite and Si and generate an electrostatic screening force among the graphite layers [14]. Templates other than PDMS were also studied for the transfer and immobilization of graphene onto predefined areas of substrate surface [5]. 31 Alternatively, patterning could also be integrated in the growth or deposition process. Specifically for graphene synthesized via chemical vapour deposition (CVD), the metal precursor coated on Si/SiO2 substrate could be pre-patterned via chemical means to grow CVD graphene in specific areas and patterns [15]. For solution-based graphene derivatives, namely GO, reduced GO and functionalized GO, ink-jet printing is a versatile and up-scalable method of patterning [16]. Most of the above mentioned methods of patterning employ additional materials such as photoresist, evaporated metal or polymer for the definition of patterns. Therefore an inevitable introduction of impurities and residual polymers would contaminate the graphene surface. Non-contact patterning based on direct writing process for the patterning of graphene, on the other hand, reduces the number of masking or photoresist steps needed when designing an intergrated circuit based on all-carbon electronics. It is also fluidic, up-scalable and low-cost. Photothermal reduction of graphene oxide provides an easy means of direct patterning of graphene oxide thin films. In fact it is so easy that Cote et al. managed to pattern graphite oxide paper with a camera flash[17]. More precise patterning would employ focused laser beam[7] or pulsed femto-second laser[18]. The resolution and scale of the patterns is limited by the instruments of optical focusing and mechanical movement. Current works are mostly still in the micrometer range [18, 19]. In our lab, we use focused laser beam technique to construct an extended area of micro-patterned GO and reduced GO multilayers on quartz, Si or Si/SiO2 substrates. For patterning purpose, a solid-state diode laser of wavelength 532 nm was focused by a Leica optical microscope onto the sample, as shown in Figure 3.1, with a 100× objective lens. A charge-coupled device (CCD) camera was fixed to the microscope for monitoring of the patterning process. The focused spot-size can be down to ~1μm in diameter. The irradiation power after focusing ranged from 1mW to a few hundred mW. The samples were placed on a computer-controlled x,y-movable stage and typical speed employed range from 10 to 50 μms-1. The experiments were mostly carried out in ambient environment, though vacuum or liquid chambers could be placed on the sample stage whenever necessary. By moving the sample on the 32 movable stage with appropriate programme, focused laser would scan over required distance or area. Removal of GO from the substrate in certain cases was achieved by sonication of the sample in deionized water for an extended period. Figure 3.1A) Schematics of the Laser Beam System to focus laser onto sample placed on the computer controlled x-y stage; B) Schematic diagram depicting focused laser beam following onto the GO film on substrate with pre-deposited gold electrodes for electrical measurement. 33 3.3 Electrical Measurement The extent of reduction of the graphene oxide can be reflected by the electronic properties of the materials. The most commonly compared parameter is the bulk conductivity ( : Sm-1) but other related values such as sheet resistance ( and sheet conductance ( - - ) ) were also reported. Sheet resistance is a measure of the electrical resistance of the sheet, independent of its thickness. It is related to bulk conductivity by the equation , where is sample thickness. Currently the best reported value for laser-reduced GO is 100-500 Ωsq-1[18]. Electrical measurements in our experiments were mostly carried out using Cr-Au electrodes, 100nm in thickness, pre-deposited onto the substrate in the two-electrode configuration by electron beam lithography. Current-voltage behaviour of the sample could be measured and the electronic properties related could be calculated from there. Unlike four-electrode configuration, two-electrode measurement is subjected to the influence of contact resistance. For graphene-gold contact however, there is negligible potential barrier or doping effect without annealing [20, 21] and therefore two electrode measurements is sufficient for room-temperature measurement. Electrical measurements were carried out by the Cascade CMPS-888 probe station connected to Keithley 6340 source meter. 34 3.4 Raman Spectroscopy First observed in 1928 by Raman and Krishnan [22], Raman spectroscopy is a relatively easy, non-destructive, non-contacting and quick measurement method to probe the inelastic scattering of light from a sample surface at room temperature at ambient pressure. A single frequency of radiation is used to irradiate the sample. The spectrometer detects scattered light with respect to its energy difference from the incident beam. The spectrum measured is used mainly to detect vibration states in the molecules to provide information on the chemical structure of substances. Raman spectra in our experiments were recorded by WITEC CRM200 Raman System in back scattering geometry from various regions of the GO film. The excitation wavelength, λ=532 nm and the spot size is ~500 nm. 3.4.1 Basic Principles When light is incident on matter, the comprising photons may pass straight through without interaction, or may be absorbed or scattered by the molecules. Absorption takes place when the photon energy matches exactly the energy gap between the ground state of a molecule and an excited state, whereby the molecule is promoted to the excited state. The absorption spectrum can be measured by comparing the incident and transmitted intensity over all wavelengths. Molecules in an excited state will eventually decay to lower energy levels and emit photons with photon energy matching the energy gaps, which can also be recorded as the emission spectrum. Absorption or emission spectroscopy can be applied over a wide range of wavelengths to determine energy levels of a molecule. The energy of the spectra is usually discussed in terms of the frequency (f) or wavenumber (ω) scales given below. On the other hand, scattering of a photon occurs when the photon energy does not exactly match energy gaps between ground level and excited levels of the molecule. Rather, the scattering process can be understood as absorption of the photon and re- 35 emission of another via a virtual state created from the polarization of electron by the incident light, the energy of which is determined by the frequency of the light source used. The scattering of the photon can be elastic or inelastic. If only electron cloud distortion is involved during the scattering process, the scattered photon will have a very small frequency change. The scattering process is regarded as elastic, and called Rayleigh scattering. However, in the event that nuclear motion is induced during the scattering process, energy will be transferred between the photon and the molecule, which is much heavier. The scattering is therefore inelastic, and the energy of the scattered photon differs from the incident photon. This is Raman scattering. Rayleigh scattering is the dominating process, and only a very small fraction of photons will be Raman scattered. Figure 3.2 Basic principle of Raman Scattering Figure 3.2 shows the basic principles involved in various scattering processes. The incident photon with energy Es is absorbed in promoting the molecule into the virtual state. The extra energy is emitted again in the form of a scattered photon with energy EL. If the molecule returns back to the original state, the scattered photon has the same energy as the incident photon and Rayleigh scattering takes place. The 36 Rayleigh process will be the most intense process since most photons scatter this way. Therefore filters need to be installed in front of the spectrometer to remove spectrum of the incident light in order to obtain information about the energy shift in the weaker scattering processes. When a molecule promoted from the ground vibrational state returns to an excited vibrational state above, it is called Stokes scattering. The scattered photon will have one vibrational unit less of energy from the incident photon. At room temperature, most molecules, but not all, are present in the lowest vibrational level. However, thermal energy would allow some molecules in an excited state following Boltzmann distribution. Scattering from these molecules as they decay back to the ground state is called anti-Stokes scattering whereby energy of one vibrational unit is transferred to the scattered photon. At room temperature, the number of molecules in an excited state is small, and therefore anti-Stokes scattering will be weaker. Usually Raman scattering is recorded only on the low-energy side for Stokes scattering but occasionally anti-Stokes scattering is preferred, for the instance when fluorescence interferes with the spectrum in the low-energy range. Raman spectrum indicates the shift in energy from that of the exciting radiation, but for simplicity the convention is still to express spectrum in cm-1. Intensities of the bands in the Raman spectrum depends on the nature of the vibration studied as well as the instrumentation and sampling factors. It is therefore more sensible to compare the relative intensities of the bands in the same spectrum, or spectra taken under the identical condition. 3.4.2 Vibrational States Molecules with N atoms have 3N-6 vibrational degrees of freedom, with the exception of linear molecules which have one more. The vibrational frequency is dependent on the mass of the atoms and the strength of the bonds. Not all vibrational modes, however, are Raman active. The electron cloud around the atoms will alter during vibration due to the shift in position of the positive nuclei. Depending on the nature of the change, vibrations may lead to a change of dipole moment or polarization. The basic selection rule is that vibrations that change the polarizability 37 of the electron cloud would give more intense Raman peaks; while vibrations that change the dipole moment would be more susceptible for infra-red absorption. Usually symmetrical vibrations fall into the former category and asymmetric vibrations the latter. For small molecules, it is possible to calculate all the vibration modes and the corresponding electron cloud change. It is difficult to apply the same analysis to a more complex molecule. Commonly the displacements need to be broken down into a number of characteristic features and assign the vibration to more than one molecule. Two or more bonds which are close together and are of similar energies can interact and the vibration of the this group of atoms will be observed in the spectrum, e.g. –CH2, -CH3 or C6H6. If the atoms are well separated or if there is a large difference in energy between the vibrations of different bonds, they can be treated separately. Energy ranges of the characteristic frequencies, as well as relative intensities of the specific peaks for the most common groups can be found in digital libraries. There is also software to compare spectrum with known standards. These methods allow the application of Raman in studying macromolecules with unknown functional groups, such as GO. However, interpretation of Raman spectrum in quantitative details, or decipher of the specific mechanisms of individual peaks or shifts require much more detailed calculations. 3.4.3 Raman Spectrum of Graphite-based Materials Raman spectroscopy has historically been used to probe structural and electronic characteristics of graphitic materials, such as carbon fibre[23], pyrolytic graphite [24], nanographite ribbons [25], fullerenes[26] and carbon nanotubes[27]. Raman spectra have been shown to provide useful information on the basic structural properties of graphene such as in-plane crystallite size of the sp2 carbon atoms [23, 24] as well as the out-of-plane stacking order [28-30]. For materials with substantially increased sp3 defects, significant changes would appear in the Raman profiles [31]. The more recent developments allow very accurate structural analysis of nano-graphite, with the establishment of an empirical formula for the in- and outof-plane crystalline size dependence of the Raman scattering intensity[32] and more exotic Raman-based information such as the atomic structure at graphite edges[33] 38 and the identification of single versus multi-graphene layers [34, 35]. Even electron or hole doping can be monitored by stiffening and sharpening of the G band [36]. Once established, this fundamental knowledge provides powerful machinery to understand newer forms of practical sp2 carbon materials. The most prominent features in the Raman spectra of graphitic materials include the G band appearing at 1582 cm-1, the D band [23, 24] at about 1350 cm -1, the D’-band at about 1620 cm-1 and the G’ band at about 2700 cm-1 as measured under laser excitation of 514nm[37]. The G band is a doubly degenerate (iTO and LO) phonon mode at the BZ center that is Raman active for sp2 carbon network. The G-band of graphite material is a doubly degenerate (TO and LO) phonon mode (E2g symmetry) at the Brillouin zone center [24] whereas the D-band is due to phonon branches around the K point and requires a defect for its activation[38].If we see the G band in the Raman spectra, we can say that the sample contains sp2 carbon networks. The D and D’ bands are defect-induced Raman features, and the integrated intensity ratio for the D band and G band is widely used for characterizing the defect quantity in graphitic materials. The evolution of the G’-band for different graphene sheets has been used to determine graphene thickness, and to locate monolayer graphene[34]. The seminal work on the Raman study of graphitic material is none other than that of Tuinstra and Koeing in 1974 [24]. In this work, the authors carried out Raman spectroscopy on materials over one whole range from amorphous carbon black to natural graphite. It was found that natural graphite gives Raman line only at 1575 cm1 , called the G-band; while other materials give a second line near 1355 cm-1, later named the D-band. The relative intensity of D-band to G-band depends on the type of material. Graphite consists of sp2 carbon hexagonal networks of carbon atoms with covalent bonding between C atoms within a plane and a weak van der Waals interaction between planes. All graphite materials have some defects in practice, and different types of materials have different types of defects. To interpret their Raman spectra, understanding of the phonon dispersion is essential. 39 For single layer of graphite, or graphene, there are two atoms in the unit cell, thus six phonon dispersion relations are obtained by calculation[37] as well as experiment[39], in which three are acoustic modes and three are optic modes. For the three acoustic and three optic phonon modes, one is out-of-plane, the other two inplane modes are longitudinal and transverse respectively. Raman activity for a crystal can only be observed in the limit where the wave vector k=0, corresponding to the totally symmetric mode of the translation lattice. Therefore only the E2g mode, corresponding to the degenerate LO and iTO phonon mode at the Γ point, is to be Raman active as fundamentals. The G-band present in all graphitic materials is therefore attributed to E2g vibration of the sp2 network. The symmetry of the E2g modes restricts the motion of the atoms to the plane of the carbon atoms. The degeneracy of the LO and iTO phonons disappears for graphite. There are two E2g modes corresponding to the atoms in two layers vibrating in opposite directions, but they still appear as one single G-band as their frequencies do not differ much. This band does not depend on the mutual arrangement of the graphite planes. The interpretation of the D band has undergone a debate lasting several decades. Tuinstra and Koenig [24] ruled out tetrahedral bonded carbon, or sp3 carbon, as the source of the D band. They also observed its linear relationship with , where is crystallite size obtained from x-ray data. Their further attribution of D band to an A1g breathing mode at K “which were inactive in the infinite lattice”, however, was later challenged as it does not account the dispersive nature of the D-band. It was observed that the D band frequency laser linearly with increases with the energy of the incident 50 cm-1/eV [40], almost independent of material type. Eventually the activation mechanism was idenditied as being due to a double resonance [41]. The scattering processes consist of one elastic scattering event by defects and one inelastic scattering event by emitting or absorbing a phonon. Once established that this peak is attributed to phonon branches around K, its dispersion with excitation energy will depend on the precise electronic structure of graphene near the Fermi energy [42]. It was also proven from confocal Raman images that Dband intensity is localized where the crystalline structure is not perfect[37]. 40 The G’-band in the range 2500-2800 cm-1 corresponds to the overtone of the D band, and therefore also called 2D or D* band. This band is the second most intense feature in the Raman spectrum of the completely ordered graphite. The evolution of G’-band was associated with stacking order of crystalline graphite [28, 29] and later quantitatively systematized to indicate the 2-D to 3-D evolution of single to a few graphene layers [43]. 3.4.4 Raman Spectra for Defective Graphite There is a myriad of extensive research on Raman study of disorders and defects in graphene-like materials. The precise nature of the disorder and defects and the complete theory for the Raman intensity of G and D peaks is still the subject of ongoing research. However in our works here, we mainly use Raman for two aspects: the identification of graphitic materials and the calculation of crystallite size. As explained previously, the earliest work on Raman spectrum of graphite has already observed the correlation , where the minimum An empirical expression which allows the determination of further developed by Knight and White [44] as[32] verified was ~2nm. from ratio was where is the integrated intensity ratio instead of peak ratio. As uniform cluster size is assumed for a system with varying grain size, the above models underestimate La due to the dominant effect of small crystallites. Actual graphitic materials are in general poly-crystalline with a crystallite size that can be controlled by heat treatment. Since electrical resistivity arises in part from the hopping of carriers between crystallites in the sample, increases in the crystallite size reduces the resistivity. A large crystallite size is also important for high capacity for applications like intercalated graphene batteries, though the crystallite size can be reduced after many charge-discharge cycles. Raman spectra of carbon films are dominated by the sp2 sites even for highly sp3 amorphous carbon samples as visible excitation always resonates with the π-states. 41 Therefore Ferrari et al. [38] considered the various factors affecting the Raman spectrum: the cluster of sp2 phase; bond disorder; presence of sp2 rings or chains; the sp2/sp3 and defined an amorphization trajectory leading from graphite to amorphous carbons [31, 45]. The disorders could be classified in three stages: i) from graphite to nanocrystalline graphite, where the Raman features are similar to those explained earlier; ii)from nanocrystalline graphite to mainly sp2 amorphous carbon, the G peak decreases from 1600 to ~1510 cm-1 and well-defined second-order Raman peaks are absent; iii) amorphous carbon, where clusters become smaller and the rings fewer and more distorted, thus the development of a D peak indicates ordering, opposite to the case of graphite. Tuinstra-Koenig model and its derivatives apply only for the first class. For class (ii) the number of ordered rings, approaches 0; for class (iii) , where is proportianl to M, . 3.4.5 Substrate Effect It has been experimentally shown that exfoliated graphene on different substrates showed little difference in their spectra [46, 47], as weak Van de Waals force between graphene sheets and the substrates play a negligible role in affecting the Raman features of graphene sheets. Epitaxial graphene on SiC, however, shows strong blueshift of G- and 2D bands, which can be explained by the interfacial carbon layer between SiC and epitaxial graphene, which is covalently bonded to the SiC substrate. The covalent bonds lead to changes in the lattice constant of graphene. The lattice mismatch between graphene lattice and the interfacial carbon layer results in a compressive stress and hence the shift of the G-band Raman peak frequencies [48]. Similarly other studies of graphene on sapphire showed blue shift of the Raman G-band due to possible carbon-sapphire bondings[47]. 42 3.5 X-ray Photoemission Spectroscopy Chemical change induced by laser irradiation was studied via micro-Raman spectroscopy and X-ray Photoelectron Spectroscopy (XPS). The XPS analysis of the sample was carried out using the Thermo Scientific Theta Probe XPS. Monochromatic Al KX-ray (1486.6eV) was employed for analysis with an incident angle of 30°with respect to surface normal. Photoelectrons were collected at a takeoff angle of 50°with respect to surface normal. The analysis area was approximately 400 µm in diameter while the maximum analysis depth lies in the range of 4 – 10 nm. C1s, O1s and valence band (VB) high resolution spectra were recorded with a pass-energy of 40 eV and a step size of 0.1 eV. 3.6 AFM The atomic force microscopy (AFM) is employed for morphological study of the sample surface. Detection of sample surface is carried out via a very fine tip on a silicon or silicon nitride cantilever which deflects upon approaching the tip to the sample surface. The deflection is measured by reflecting a laser spot from the top surface of the cantilever. All experiments reported here were carried out in the tapping mode whereby the tip does not come into contact with the sample surface. Rather, the cantilever oscillates up and down at a frequency close to resonance with an amplitude of 100~200nm. Due to the forces of interaction between the tip and the sample surface at close range, the closer the tip the smaller the amplitude would be. The cantilever scans over the sample with adjusted height to maintain fixed amplitude of oscillation, and therefore generate a height profile of the sample surface. Tapping mode reduces damage to the sample surface of the tip as compared to contact mode. AFM measurement henceforth reported was carried out under ambient conditions with a Veeco metrology 3100 Nanoscope IV apparatus in the tapping mode of operation using silicon cantilevers with spring constant of 20~100 N/m and typical resonance frequencies between 200 to 250 kHz. Imaging was captured in the attractive regime of tip-sample interaction, recording height (topography) images. 43 In addition, AFM was also reported in literature as means for nano-scale patterning of graphene oxide using a heated AFM tip[49]. 3.7 Microscopy and Spectrometer Unless otherwise specified, optical micrograph reported were taken with Olympus BX51 microscope attached with DP73 camera illuminated under a 100W halogen lamp, with up to 100x magnifications. Spectrum measurement was taken by feeding the microscope image to a Mightex high-resolution high-stability CCD spectrometer (HRS-Series). Spectrum was processed by Mightex software to remove dark background and OriginLab with fast-fourier transform smoothening function. 44 References [1] N. I. Kovtyukhova et al., Chemistry of Materials 11, 771 (1999). [2] N. A. Kotov, I. Dekany, and J. H. Fendler, Advanced Materials 8, 637 (1996). [3] T. Cassagneau, and J. H. Fendler, Advanced Materials 10, 877 (1998). [4] H. A. Becerril et al., Acs Nano 2, 463 (2008). [5] Z. Q. Wei, D. E. Barlow, and P. E. Sheehan, Nano Letters 8, 3141 (2008). [6] S. Wang et al., J. Am. Chem. Soc. 131, 16832 (2009). [7] Y. Zhou et al., Advanced Materials 22, 67 (2010). [8] L. J. Cote, F. Kim, and J. X. Huang, J. Am. Chem. Soc. 131, 1043 (2009). [9] C. X. Cong et al., Journal of Physical Chemistry C 113, 6529 (2009). [10] S. P. Pang et al., Advanced Materials 21, 3488 (2009). [11] M. J. Allen et al., Advanced Materials 21, 2098 (2009). [12] T. R. Hendricks et al., Advanced Materials 20, 2008 (2008). [13] X. Liang, Z. Fu, and S. Y. Chou, Nano Lett. 7, 3840 (2007). [14] X. Liang et al., Nano Letters 9, 467 (2009). [15] C. A. Di et al., Advanced Materials 20, 3289 (2008). [16] V. Dua et al., Angewandte Chemie-International Edition 49, 2154 (2010). [17] L. J. Cote, R. Cruz-Silva, and J. X. Huang, Journal of the American Chemical Society 131, 11027 (2009). [18] Y. L. Zhang et al., Nano Today 5, 15 (2010). [19] J. J. Liang et al., Acs Applied Materials & Interfaces 2, 3310 (2010). [20] R. S. Sundaram et al., Applied Physics Letters 95 (2009). [21] R. S. Sundaram et al., Nano Lett. 11, 3833 (2011). [22] C. V. Raman, and K. S. Krishnan, Nature 121, 501 (1928). [23] F. Tuinstra, and J. L. Koenig, Journal of Composite Materials 4, 492 (1970). [24] F. Tuinstra, and J. L. Koenig, Journal of Chemical Physics 53, 1126 (1970). [25] L. G. Cancado et al., Physical Review Letters 93 (2004). [26] D. S. Bethune et al., Chemical Physics Letters 179, 181 (1991). [27] H. Hiura et al., Chemical Physics Letters 202, 509 (1993). [28] P. Lespade, R. Aljishi, and M. S. Dresselhaus, Carbon 20, 427 (1982). [29] P. Lespade et al., Carbon 22, 375 (1984). 45 [30] H. Wilhelm et al., Journal of Applied Physics 84, 6552 (1998). [31] A. C. Ferrari, and J. Robertson, Physical Review B 61, 14095 (2000). [32] L. G. Cancado et al., Applied Physics Letters 88, 3 (2006). [33] L. G. Cancado et al., Physical Review Letters 93 (2004). [34] A. C. Ferrari et al., Physical Review Letters 97 (2006). [35] A. Gupta et al., Nano Lett. 6, 2667 (2006). [36] J. Yan et al., Physical Review Letters 98 (2007). [37] M. A. Pimenta et al., Physical Chemistry Chemical Physics 9, 1276 (2007). [38] A. C. Ferrari, Solid State Commun. 143, 47 (2007). [39] J. Maultzsch et al., Physical Review Letters 92, 4 (2004). [40] R. P. Vidano et al., Solid State Commun. 39, 341 (1981). [41] C. Thomsen, and S. Reich, Physical Review Letters 85, 5214 (2000). [42] R. Saito et al., New Journal of Physics 5 (2003). [43] L. G. Cancado et al., Carbon 46, 272 (2008). [44] D. S. Knight, and W. B. White, J. Mater. Res. 4, 385 (1989). [45] A. C. Ferrari, and J. Robertson, Physical Review B 64 (2001). [46] Y. Y. Wang et al., Journal of Physical Chemistry C 112, 10637 (2008). [47] I. Calizo et al., Applied Physics Letters 91 (2007). [48] Z. H. Ni et al., Physical Review B 77 (2008). [49] Z. Q. Wei et al., Science 328, 1373 (2010). 46 Chapter 4: Photothermal Reduction 4.1 Introduction As explained in earlier chapters, graphene oxide (GO) thin film can be processed via photothermal reduction under irradiation of focused laser beam. In our experiments we demonstrated the localized reduction of GO into reduced graphene oxide (rGO) thin film using a focused laser beam system. Patterning of rGO structure in a GO matrix was also demonstrated. The product of reduction was studied for the change in its morphological, chemical and electrical properties. Based on our experimental studies, mechanism of the photothermal reduction was also proposed. In this chapter, we present systematic investigations on the localized reduction of thin GO film via direct focused laser beam irradiation. We have achieved reduction of GO by irradiating with a focused laser beam under ambient conditions. A hundred-fold improvement in the conductivity of essentially insulating GO film after laser irradiation was observed. The electrical properties of laser reduced channels were found to be tunable with gate voltage and mobility was estimated to be in the range of 1-10 cm2/Vs. X-ray photoelectron spectroscopy and Raman spectroscopy studies revealed the presence of rGO and further electrical measurements confirmed the rGO formation at the laser irradiated region. Partial oxidative burning combined with photothermal reduction was identified as the underlying mechanism for the conductivity enhancement after laser irradiation on GO film. Though reduction of film thickness was due to both oxidative burning and photothermal reduction, we demonstrate that the former can be minimized through control of film thickness and laser power. Unlike previous work, we were able to remove the surrounding GO and isolate a stand-alone rGO conducting channel and therefore achieving a reproducible, fast and simple method for patterning as well as to induce insulator-conductor transformation in GO. 47 4.2 Conductivity change High-quality GO was prepared based on modified Hummer’s method[1] as described in Section 2.2.2 of Chapter 2. The GO suspension was spin-coated on a highly doped silicon wafer with a 100 nm SiO2 dielectric layer. Au/Cr electrodes with approximately 10 μm separation were patterned onto the wafer via photolithography prior to GO deposition. The deposited thin film was visible under optical microscope (Figure 4.1A insert). Laser pruning of GO was carried out using a focused laser beam setup as detailed in Figure 3.1 in Chapter 3. A continuous wave laser beam was focused to a diffraction limited spot size of [...]... both methods reported here allow the localized reduction of GO to RGO on a deposited GO film, with the best resolution of ~1μm Therefore 1 patterning and reduction of GO was carried out simultaneously instead of the traditional methods of patterning GO thin film via oxidative removal before reduction As the electronic properties of GO and RGO differs drastically, these green methods for localized reduction. .. oxide (RGO) is the product from the reduction of graphene oxide (GO) Reduction is one of the most important reactions of graphene oxide as it restores the disrupted sp2 bonding network of GO, in order to recover graphene- like electrical properties due to the π-network The product of reduction is also called highly-reduced graphene oxide (HRG), and chemically derived graphene (CDG) in literaures However... properties, methods of synthesis and applications The current methods for reduction and patterning of GO is also presented, as well as some theories on the electrical conduction in RGO In Chapter 3, the experimental methods for the synthesis, reduction and characterization of GO film is described Some theories on Raman spectroscopy of graphitic material were also detailed In Chapter 4, methods of photothermal. .. sonication of fine GO powder[59] or by serial dilution of an aqueous dispersion of aqueous graphene oxide with appropriate organic solvent into a primarily organic media[29] 12 2.2.3.2 Functionalization One key application of graphene oxide (GO) is as a precursor to synthesize reduced graphene oxide (RGO) via removal of oxygen-containing functional groups in order to restore the structure and properties of graphene, ... reduction of GO leads us one-step closer to achieving continuous carbon electronics Via investigation of the properties of RGO produced as well as the reduction process, we strive to better understand the mechanisms of these methods for its future optimizations and applications This thesis is organized as follows In the next chapter, some backgrounds on graphene, graphene oxide and reduced graphene oxide. .. discovery and since then attracted remarkable research interests A major hurdle in research and application of graphene is to find an efficient method for large-scale synthesis of the high-quality material One of the potential methods explored was the chemical exfoliation of graphite via oxidation -reduction cycle The oxidation of graphite [2] produces graphene oxide (GO), or earlier known as graphite oxide. .. deposition (CVD) of graphene monolayers[19, 20]; and longitudinal unzipping of carbon nanotubes (CNTs)[21-23] Chemical exfoliation of graphite via oxidation -reduction cycle was, in the early stage, one of the potential methods for producing graphene in a cost-effective and up-scalable manner This advantage was less significant with the development of CVD methods to produce large-scale graphene rapidly... evolution of CO and CO2 in agreement with the microscopy observations They also found that residual oxygen in fully reduced GO is a consequence of the formation of highly stable carbonyl and ether groups that cannot be removed without destroying the graphene basal plane These calculations confirm and explain the difficulties in restoring sp2 structures of RGO 14 2.3.2 Reduction of Graphene Oxide Reduction of. .. hybridization The honeycomb lattice of carbon atoms has been confirmed by transmission electron microscopy Rippling of the flat graphene monolayer is present in both suspended graphene or graphene on a substrate, which is believed to compensate for the instability of 2D crystals [2] 2.1.1.1 Band Structure The calculation of energy band of graphene is the same as that of a single layer of graphite, ignoring inter-layer... react with water, making it perfect for GO reduction One of the disadvantages of using chemical methods of reduction, hydrazine in particular, is the introduction of heteroatomic impurities While effective at removing oxygen functional groups, nitrogen tends to remain covalently bonded to the surface of graphene oxide and affect the electronic structure of the graphene Later sodium borohydride (NaBH4) ... structures of RGO 14 2.3.2 Reduction of Graphene Oxide Reduction of GO to graphene can be carried out via a number of different approaches such as thermal[75], chemical[76] and electrochemical methods[ 77]... Reduced Graphene Oxide As one can tell from the name, reduced graphene oxide (RGO) is the product from the reduction of graphene oxide (GO) Reduction is one of the most important reactions of graphene. .. photothermal reduction and patterning of GO is detailed as well as characterization of the properties of GO and RGO In Chapter 5, electrochemical reduction of GO is detailed with a focus on the reduction