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Quantum Dots Dr. Ricardo Faccio Centro NanoMat/DETEMA Facultad de Química Universidad de la República Noviembre de 2020 rfaccio@fq.edu.uy 1 1 Punteo 1. Introducción 2. Definición de Quantum Dots 3. Absorción Óptica y Fotoluminiscencia 4. Grafeno y Materiales Carbonosos 1. GQD y GOQD 5. Propiedades Ópticas 6. Algunas Aplicaciones Biomédicas. 7. Conclusiones finales. Noviembre de 2020 2 2 1
1. ¿Qué es la Nanotecnología? p Es el estudio y control de la materia, a una escala entre 1 y 100 nm 1 nm= 1/1 000 000 000 m 1 nm = 1x10-9 m La nanociencia es el estudio de fenómenos y manipulación de materiales a escalas atómica, molecular y macromolecular; dónde las propiedades difieren significativamente de aquellas a gran escala. La nanotecnología es el diseño, la caracterización, la producción y la aplicación de estructuras, dispositivos y sistemas para controlar la forma y tamaño a escala nanométrica. Noviembre de 2020 3 3 1. Conceptos: Propiedades Dependen del tamaño Confinamiento cuántico Propiedades ópticas Cristal micrométrico CdSe Nanocristal Noviembre de 2020 4 4 2
1. Conceptos: Metodologías: Top- Down & Bottom-Up p Top-down: Reducción de tamaño. Los mecanismos y las estructuras se miniaturizan a escala nanométrica. Este tipo de Nanotecnología ha sido el más frecuente hasta la fecha. p Bottom-Up: Auto ensamblado. Se comienza con una estructura nanométrica como una molécula y mediante un proceso de montaje o auto-ensamblado, se produce el crecimiento. Este enfoque, que algunos consideran como el único y "verdadero" enfoque nanotecnológico, ha de permitir que la materia pueda controlarse de manera extremadamente precisa. Noviembre de 2020 5 5 1. Top-Down & Bottom-Up Noviembre de 2020 6 6 3
1. Estructura Electrónica y Dimensionalidad p Evolución de2 moléculas a nanopartículas y Size Effects on Semiconductor Nanoparticles 29 luego a sistemas “bulk” HOMO → Fig. 2.12 Evolution of theBanda destructure energy level Valenciafrom a hypothetical diatomic molecule (extreme LUMO Banda deright). Conducción Enc 0 left) to a bulk→semiconductor (extreme g and Eg indicate the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) Energía de Gap: for Diferencia a nanocrystal and bulk,de energía respectively (CB entre VBband, = conduction y CB VB = valence band). Reprinted Noviembre de 2020 with permission from Ref. [45] 7 Niveles Electrónicos menos discretos The same approach can be extended to larger molecules, clusters and even bulk 7 materials. As the molecule (e.g, a small CdSe cluster) becomes larger, the number of AOs that are combined to form MOs (bonding and anti-bonding) increases, leading to an increasingly larger number of energy levels and decreasing the HOMO-LUMO energy gap (Fig. 2.12). Each MO combination has a well-defined energy value, but MOs with intermediate energy values are more common than those with energies near the minimum or maximum values (i.e., fully bonding or fully anti-bonding). This means that the density of MO states is maximum at 22 intermediate R.energy Koole et al. values, decreasing to a minimum at both energy extremes (i.e., 20 R. Koole et al. highest −V (finite) for r < D/2 and zero elsewhere. Inserting 0 Eq. 2.11and in thelowest). Schrödinger For a sufficiently large number of combining atoms (i.e., when equation gives the solutions for the discrete energy-levels of a confined electron in a sphere [4, 5]: the bulk limit is reached) the energy levels become so numerous and so closely Fig. 2.5 Schematic representation of the 2 2 hspaced 2! v that a promotion quasi-continuum (i.e., an energy band) is formed, analogous to the conf E ðDÞ ¼ n;l nl 2 ð2:12Þ of an electron from 1. Propiedades Ópticas $ m D conduction and valence the valence bandbands to the described above. The HOMO level is the top of the * where m is the effective mass of electrons (or holes), and χ are the roots conduction of the band in a direct VB, onwhereas the LUMO is theas bottom of the CB (Fig. 2.12). nl Bessel function, which are absolute values depending (and increasingband with) gap the semiconductor principal quantum numbers n (1, 2, 3,…) and azimuthal quantum number l (0, 1, 2, 3…, corresponding to s, p, d,…, orbitals) (Fig. 2.6). A result of the absorption of a semiconductor The lowest energy level (n = 1, photon [5, NC 10]. can be Reprinted with regarded as a very large molecule or cluster con- l = 0) has the symmetry of a 1 s orbital in a hydrogen atom (i.e., it is a 1S level). A Ante una sisting of a few tens to a few[45] thousand atomic valence orbitals, forming as many hydrogenexcitación atom and a quantum dot is (óptica ohastérmica) es posible promover electrones direct consequence of the difference in the potential function (V(r)) permission between the from Ref. that the latter system no restriction of MOs n, as is(for the caseexample, in the hydrogen a 1.5 nm diameter CdSe NC contains about 50 atoms, while a desde atom la(l BV≤ n − 1).a la theBC. Proceso quantum number l with respect to quantum number Therefore second energy level in a quantum de dot hasabsorción quantum deja un hueco (+) en BV y 10 nm NC consists of 104 atoms). Therefore, its electronic structure will be char- un (-) en la BC. acterized by energy bands with a large density of levels at intermediate energy values and discrete energy levels near the band edges, where the density of MO states is small. Moreover, the HOMO-LUMO energy gap will be larger than for bulk and size-dependent, increasing with decreasing size of the NC (Fig. 2.12). This explains both quantum confinement effects discussed in the previous section from a molecular point-of-view. The MO approach described above provides a simple and general description of where the second term contains Hydrogen-like set of energy levels in which Ry* is the electronic the structure of a hypothetical exciton Rydberg 1-dimensional energy (corresponds NC,energy to the ionization and clearly illustrates of the lowest the size dependence hydrogenicofstate) the [5]. HOMO-LUMO gap. The third term in Eq. 2.8However, accounts for Fig. 2.12energy the kinetic presents of the the exciton centre of mass motion in which k is the exciton wave vector. The kinetic energy term resembles the dispersion curve of a free electron, but with the differ- ence that it is corrected for the effective mass of the exciton. The third term originates from the fact that the interacting holes and electrons can be described as Fig. 2.6 Schematic of the effect of quantum confinement on the electronic structure two ofparticles a semiconductor. The arrows indicate the lowest energy absorption transition. a Bulk semiconductor interacting via a Coulomb potential. A Hamiltonian analogous to the (CB = conduction band; VB = valence band). b Three lowest electron (E ) and hole hydrogen e nl h nl (E ) energy atom Hamiltonian is used to calculate the energy of the exciton, in which Se genera un Excitón, que consiste eneffective un par mass electrón-hueco levels in a quantum dot. The corresponding wave functions are represented by dashed lines. c Semiconductor nanocrystal (quantum dot). Reprinted with permission from [45] the exciton replaces the free electron mass m0. By analogy to the Hydrogen atom the most probable distance between the electron and hole in an interactuando electrostáticamente. La distancia de interacción se exciton is given by the so-called exciton Bohr radius (a0) [5] denomina Radio de Bohr del excitón. ! " Noviembre de 2020 h!2 e 1 1 8 a0 ¼ þ ð2:9Þ Radio de Bohr del Excitón e2 me " mh " where me* and m*h are the effective masses of electron and hole, respectively. 8 Further, e is the electron charge and ε is the dielectric constant of the semicon- ductor. The exciton Bohr radius provides a very useful length scale to describe the spatial extension of excitons in semiconductors, and ranges from *2 to *50 nm depending on the semiconductor [3, 5]. It is interesting to note that the exciton Bohr radius a0 and the band gap of the semiconductor are correlated, so that materials with wider band gaps possess smaller a0 (e.g., Eg and a0 are, respectively, 0.26 eV and 46 nm for PbSe, 1.75 eV and 4.9 nm for CdSe, and 3.7 eV and 1.5 nm for ZnS). 4
1. Confinamiento Cuántico Cuando el Radio de Bohr del excitón está totalmente confinado dentro de la nanoestructura, el efecto del confinamiento 28 es superior, y se tiene un “Quantum Dot”. R. Koole et al. Fig. Noviembre de 2020 2.11 Schematic illustration of the energy level structure of semiconductor nanostructures 9 Esfera with reduced negra representa dimensionality(2D, la esfera 1D and 0D indicate two, con radio one, or zero-dimensional, de Bohr respectively) [3]. The energy level structure of a bulk semiconductor (3D) is shown for comparison. The exciton Bohr diameter is represented by the sphere. DOS gives the density of states 9 only in the thickness direction, a Quantum Well is formed (1-dimensional con- finement). A schematic overview of the energy level structure of semiconductor nanostructures with reduced dimensionality is given in Fig. 2.11. 2.4.2 Nanocrystal as a Large Molecule: Building Up Atom by Atom 1. Confinamiento cuántico y Another method to explain the unique properties of a quantum dot is based on a bottom-up approach [3, 16]. In this approach, the QD is seen as a large molecule or cluster. In analogy with quantum chemical methods for obtaining molecular orbitals Propiedades Ópticas (i.e. the Linear Combination of Atomic Orbitals, LCAO), the overall wave func- tions in a QD can be constructed24from the individual atomic orbitals [3]. The simplest example of a multiple electron molecule is that of diatomic R. Koole et al. hydrogen (H2). In this molecule, two atomic orbitals (AOs) combine to form two molecular orbitals (MOs) that spread out over both H atoms, namely a bonding and an anti-bonding MO. The bonding MO is lower in energy compared to the indi- vidual AOs, whereas the anti-bonding MO is higher in energy than the individual AOs. The MOs are occupied by electrons in such manner that the potential energy of the molecule is minimized. In the H2 molecule the 2 electrons originally in the 1s AOs of the individual H atoms are accommodated in the bonding MO, thereby leaving the anti-bonding MO unoccupied. The highest occupied molecular orbital is referred to as HOMO and the lowest unoccupied molecular orbital is called LUMO. Fig. 2.7 Schematic representation of the quantum confinement effects: the bandgap of the • Las propiedades dependen semiconductorfuertemente delsize,tamaño. material increases with decreasing and discrete energy levels arise at the band- • Egap se relaciona inversamente con el tamaño edges. Note that the energy difference between the band-edge levels also increases with decreasing size. Lower panel shows a photograph of the fluorescence of 5 dispersions of CdSe QDs with • Presencia de niveles elecrónicos discretos different sizes, under excitation with a UV-lamp in the dark. Reproduced by permission of the Royal Society of Chemistry from Ref. [2] • Noviembre de 2020Efecto del Confinamiento cuántico 10 Equations Donega,2.13 and 2.14 C.D.M. et alclearly Chem. describe the two Soc. Rev. most 40, important consequences 1512–1546 (2011) of quantum confinement. The first consequence is that the band gap of a semi- conductor NC becomes larger with decreasing size, scaling as D−2 if the Coulomb 10 interaction is negligible. The second consequence is that discrete energy levels (with different quantum numbers) arise at the band-edges of both the conduction band and valence band. These two size-dependent effects are schematically depicted in Fig. 2.7. In practice, this means that the optical band gap of QDs can be tuned by simply changing their size. For QDs emitting in the visible (e.g.,CdTe or CdSe QDs) this is nicely visualized by their size-dependent luminescence colours (Figs. 2.7 and 2.8). Quantum confinement effects are also reflected in the optical absorption spectra of QDs. This is illustrated in Fig. 2.8, which shows the absorption spectra of CdTe QDs of different sizes. In the strong quantum confinement regime the energy- spacing between the discrete levels of the envelope functions with different quan- 5 tum numbers (Eq. 2.12) is in the order of hundreds of meV, and therefore optical transitions between these levels can be clearly resolved in the optical absorption
1. Confinamiento cuántico Confinamiento cuántico Propiedades ópticas Cristal micrométrico CdSe Nanocristal Noviembre de 2020 11 11 2. Definición: Quantum Dots p Definición: n Las nanopartículas que confinan sus excitones en las tres direcciones del espacio se denominan “Quantum Dots”. p Características (I): n En general son nanopartículas semiconductoras cuyas dimensiones se ubican en los rangos de 1 a 10 nm. n Átomos Artificiales: cualquier modificación produce cambios importantes (eléctrico y ópticos). n Niveles electrónicos discretos, cuasi-moleculares. n Absorción y fotoemisión bien definida. Noviembre de 2020 12 12 6
2. Definición: Quantum Dots p Características (II): n Sus propiedades ópticas dependen del tamaño: p Efecto de confinamiento cuántico. p Fotoluminiscencia p Comportamiento Eléctrico n Pueden ser Inorgánicas: p Semiconductores: CdS, CdSe, GaAs, etc. p Metálicas: pero de tamaños mucho más pequeños. n Pueden ser Orgánicas: p Graphene Quantum Dots p Graphene Oxide Quantum Dots Noviembre de 2020 13 13 3. Propiedades Ópticas p2 Absorción y Fotoluminiscencia Size Effects on Semiconductor Nanoparticles 25 Muestras expuestas a radiación UV Fig. 2.8 (Left) Absorption spectra of colloidal suspensions of CdTe nanocrystal (NC) quantum dots ofdedifferent Noviembre 2020 sizes. (Right) Photograph of vials containing colloidal suspensions of CdTe QDs 14 of different sizes under UV excitation. Reprinted with permission from Ref. [45] 14 7
3. Fotoluminiscencia (PL) p Es el proceso de emisión de Luz que sigue25 2 Size Effects on Semiconductor Nanoparticles a la absorción de fotones (PL) Noviembre de 2020 15 15 Fig. 2.8 (Left) Absorption spectra of colloidal suspensions of CdTe nanocrystal (NC) quantum dots of different sizes. (Right) Photograph of vials containing colloidal suspensions of CdTe QDs of different sizes under UV excitation. Reprinted with permission from Ref. [45] 3. Absorción Óptica y PL 2 Size Effects on Semiconductor Nanoparticles 49 p Ejemplo para CdTe coloidal Fig. 2.9 a Three lowest electron (Eenl) and hole (Ehnl) energy levels in a semiconductor nanocrystal quantum dot. The corresponding wavefunctions are represented by the dashed lines. Allowed optical transitions are given Noviembre by the arrows. b Assignment of the transitions in the absorption de 2020 16 spectrum of colloidalFig. CdTe 2.23 quantum Absorptiondots. Reprinted withspectra and photoluminescence permission from of colloidal CdTe[45] nanocrystals 16 3. The abstract of a paper published in a well-known scientific journal is repro- transition to the 1Ph to 1Pebelow: duced level, and so on (Fig. 2.9). The optical selection rules in a QD will be discussed in more Abstract: detailstructure Pronounced in Sect. 2.5. excited luminescence (XEL) has been in X-ray observed in dilute Tb-doped It is interesting to note that the exciton Bohr Y O (Y 2O3:Tb) 2 3 radius a0nanocrystals. provides aThis veryeffect affords a convenient means to assess different energy transfer mechanisms in the nanocrystals and length scale to evaluate the impact of quantum confinement on the properties of also an opportunity for novel device applications. Sharp jumps and oscillations semiconductor NCs. are As found discussed in the above, confinement XEL output beginsX-ray with the incident to affect energy the exciton around the 8 wave function as the size of the NC approaches a0. This means that the onset of absorption edges of Y and Tb. When compared with a bulk Y 2 O 3 :Tb sample,
3. Tecnología de Pantallas p LCD: n Luz Azul de fondo con filtro de fosforo para regular la intensidad de cada color: p QDLED: n Mantiene la alimentación trasera, pero no hay filtro de fosforo. n Llega más luz y cada diodo emite más intensidad y a longitud de onda más definida. n Samsung HD TV quantum dot Noviembre de 2020 17 17 D. Mombrú et al. Physica E: Low-dimensional Systems and Nanostructures xxx (xxxx) xxx-xxx Graphene Quantum Dots F OO PR D TE Fig. 7. Electronic density differences maps (EDDM) depicting the most relevant optical transitions for S-c utilizing B3LYP and CAM-B3LYP functional. Yellow regions indicate a loss of electron density in the transition to the excited state and blue regions indicate a gain of electron density in a transition to the excited state. EC Noviembre de 2020 18 RR 18 Fig. 8. Electronic density differences maps (EDDM) depicting the most relevant optical transitions for S-e utilizing B3LYP and CAM-B3LYP functional. Yellow regions indicate a loss of electron density in the transition to the excited state and blue regions indicate a gain of electron density in a transition to the excited state. CO UN Fig. 9. Electronic density differences maps (EDDM) depicting the most relevant optical 9 transitions for N-c utilizing B3LYP and CAM-B3LYP functional. Yellow regions indicate a loss of electron density in the transition to the excited state and blue regions indicate a gain of electron density in a transition to the excited state. Nitrogen atoms were indicated
4. Formas alotrópicas 19 4. Grafeno: Premio Nobel de Física 2010 Novoselov, et al Science 306 (2004),666 20 10
4. Grafeno: Propiedades p Semiconductor de Gap Nulo p Comportamiento fermiónico de RSC Advances masa cero with some of them bonded to oxygen, rath p Velocidad de portadores cercana a of graphene with surface oxidation. la de la luz Functional groups attached on GO sur mined by XPS and it reveals the nature of p Alta conductividad eléctrica bonds in their various states as unoxidized C–O, C]O, and COOH. Several XPS inve p Alta conductividad térmica conrms the peaks positions related wit p Es el material con mayor dureza groups. The deconvoluted C1s signal of Published on 28 June 2016. Downloaded by University of California - San Diego on 22/04/2017 16:45:44. conocido hasta el momento composed of mainly ve peaks (at room te positions corresponding to sp2 carbon (284.5 eV) and C atoms bonded to hydro epoxide (C–O–C, 286.55 eV), carbonyl (i Fig. 2 Representation of the procedures used to form GO starting carboxyl groups (COOH, 289.2 eV). The in with graphite flakes. Under-oxidized hydrophobic carbon material of GO shows the diminishing of the oxygen recovered during the purification of IGO, HGO, and HGO+. The peaks and a very small fraction of C increased efficiency of the IGO method is indicated by the very small amount of under-oxidized material produced.64 Reprinted (adapted) 1000 ! C.88 The other XPS analysis,38,81 co with permission from ref. 64. Copyright (2010) American Chemical lution of the C1s spectra using four com Society. C–OH, C–O–C, and COOH, while ignorin iC]O groups. In similar way, the deco information on commonly used graphite and carbons, as well as spectra contains the main peaks aroun the terminology used to describe these materials.73 533.43 eV and these peaks are assigned to bonded to aromatic carbon),75,82 C–O (ox 21 aliphatic carbon), and phenolic (oxyg 3. Functional groups on graphene aromatic carbon)58,90 groups, respectively. oxide and its analysis an additional peak at a higher binding en The XANES is the other powerful chara The structure of GO is described as graphene sheet bonded with analysis of GO materials. It provides valua several oxygen in the form of hydroxyl, carboxyl, epoxy and degree of bond hybridization in mixed s others etc. groups as shown in Fig. 3.37 Also, the structure of GO specic bonding congurations of functio depends signicantly on the purication procedures, rather of alignment of the graphitic crystal struc than, as is commonly thought, on the type of graphite used or Raman spectroscopy is an experimen oxidation protocol.74 The exact identity and distribution of oxide commonly used to characterize all sp2 c functional groups depend strongly on the extent of oxidation. zero dimensions, such as 3D graphite, 2D The appearance of chemical composition inside GO and the nanotubes, and 0D fullerenes.2 The R oxygen containing functional groups in GO can be identied displays two major D band (1340 cm"1) a 4. Grafeno: using various techniques, including X-ray photoelectron spec- troscopy (XPS),37,75–79 X-ray absorption near-edge spectroscopy (XANES),71,76,78,80,81 Fourier transform infrared spectroscopy (1580 cm"1).88 The G-band, which is ch hybridized carbon networks, originates scattering from the doubly degenerate (FTIR),78,82–84 Raman spectroscopy79,81,85 and solid-state nuclear graphite in the Brillouin zone center, w p Algunas Características del Grafeno: magnetic resonance.37,84,86,87 Mkhoyan et al.41 studied the elec- tronic and atomic structure of GO using dark eld imaging of peak comes from the structural imperfe single and multilayer sheets. The results of electron energy loss n Gap nulo spectroscopy used for measuring the structure of carbon and oxygen K-edges in a scanning transmission electron microscope n Solventes polares (por ej.: agua) indicate the high ratio of sp3 C–O bonds induces structural distortions. This suggests that the atomic structure of GO sheets p Variantes: should resemble a mostly amorphous 2D sheet of carbon atoms n Confinamiento Cuántico & Estructura de Borde p Apertura de Gap n Funcionalización: p Óxido de Grafeno (GO) Fig. 4 XPS spectra of GO and oxygen fu p Solubilidad (estabilidadFig.coloidal) 3 Structure of GO. 37 Reprinted by permission from Macmillan different temperature.76 Reprinted (adapted) w Publishers Ltd: [Nature chemistry] (ref. 37), copyright (2009). 76. Copyright (2011) American Chemical Soc n Fotoluminiscencia: p Aplicaciones Biomédicas 64996 | RSC Adv., 2016, 6, 64993–65011 This journal is © The Roya Noviembre de 2020 22 RSC Adv., 2016, 6, 64993–65011 22 11
4. Graphene Quantum Dots 224 P. Tian et al. / Materials Today Chemistry 10 (2018) 221e258 p Formación Graphene Quantum Dots. shape formation of carbon-metal nano-composites [61]. In addition, the electronic and optical properties of GQDs, such as band gap [62] and so on. While as the bottom-up approach is based growth of appropriate molecular precursors, such as small n Dimensión lateral inferior a 60 - 100 nmcules and fluorescence [63,64] etc., are dependent on their sizes and shapes. Therefore, the modulation of the intrinsic properties of [1,2]and polymers, into nano-sized GQDs by hydroth microwave-assisted hydrothermal, soft-template and n Multilayer con número de capas inferior catalyzed GQDs is dependent on the accuracy in controlling its sizes and shapes etc. Conditions of fabrication methods are keys to solving a 10 methods etc. Such bottom-up approach possess vantages, such as fewer defects and controllable of siz nm this issue. GQDs have displayed various unique phenomena, such as morphology; contrary to the top-down approach. Howev up-conversion PL [65e67]. However, the current mechanisms were bottom-up approach has suffered from poor solubility, sm p Todo esto permite: mainly acquired according to the optical behaviors leading to confusing results due to the different preparation conditions, hence size and aggregration issue etc. In the next context, detailed aration methods are presented. Bottom-up methods will b there is a lack of understanding on the exact PL mechanism. sented first and then followed by top-down. n Absorción Consequently,ensomeelforms UV of standard measurements coupled with theoretical studies should be carried out to better understand the n Fotoluminiscencia PL phenomenon. To date, most of the reported PL colors of GQDs 2.1. Bottom-up methods were ranged from blue to yellow [60]. The narrow spectral coverage As previously mentioned, bottom-up methods are ba of GQDs is limiting its applications in optoelectronic devices. The growth of the sources including graphene-like smaller po expanding of the spectral coverage of GQDs to all visible wave- aromatic hydrocarbons (PAHs) and appropriate molecule lengths and even near-infrared (NIR) is an important area of GQDs. However, this method can be subdivided into fou research in the future. Some groups [27,68,69] have obtained the routes, which cover hydrothermal method, microwave-a NIR PL spectrum by doping nitrogen into GQDs. hydrothermal method, soft-template method and metal-ca The future of GQDs is more promising once the above challenges method, according to the way external energy is provid Noviembre de 2020 1 RSC Adv., 2016, 6, 64993–65011 have been solved. In this review, we discuss the current preparation 23 the characteristics of fabrication. methods of GQDs2inMaterials Section 2, andToday Chemistry the functionalization of 10 (2018) 221e258 GQDs, such as heteroatoms doping, sizes and shapes controlling and forming its composite with other functional materials, in Section 3. 2.1.1. Hydrothermal method 23 In Section 4, some applications related to GQDs will be reviewed. Hydrothermal method involves various techniques of c Finally, a conclusion will be presented. lizing substances from high-temperature aqueous solutions vapor pressures. The fabrication of single-crystalline GQDs t the hydrothermal method has been demonstrated by 2. Preparation methods for graphene quantum dots research groups. In early 2012, Dong et al. [69] successfully cated GQDs, having a size of ~15 nm, using citric acid (CA) as According to the current fabrication methods of GQDs as re- in the hydrothermal method that produced a photolumine ported in literatures, the syntheses of GQDs can be classified into (PL) quantum yield (PLQY) of 9.0%. The mechanism of synth two main categories, namely top-down and bottom-up preparation GQDs using CA via hydrothermal method is depicted in Fig. approaches. As shown in Fig. 4, top-down approach involves direct atomic force microscopy (AFM) image of GQDs prepared b cleaving of bulk carbon materials into nanoscale GQDs via liquid and co-workers is displayed in Fig. 5b, which indicated th exfoliation and electron beam lithography techniques etc. Such GQDs were mostly nano-sheets of ~15 nm in size with heigh approach has the advantages of abundant raw materials and would range of 0.5e2.0 nm. Using CA and ethylenediamine (EDA) 4. Obtención de Grafeno y GO usually produce oxygen-containing functional groups at the edge, thus facilitating their solubility and functionalization. However, this approach has also suffered from some disadvantages, such as low bon source materials, Yang's group [70] successfully produ trogen doped GQDs (N-GQDs) with a size of 5e10 nm, exhibited relatively high PLQY of 75.2%. They suggested th yield, large density of defects, and non-controllable of size and high PLQY was possibly due to two factors: the higher prod p Top-Down y Bottom-Up yield (60%e70%), and the large number of surface defects, and edges from N-doping using EDA. Below is an equation to the PLQY, which is expressed as [75,76]: ! " #$ . % Fc ¼ Fst Kc Kst hc hst where F is QY, K is the slope determined by the curves, h refractive index of the solvent, st refers to a standard with quantum yield and c indicates unknown samples. For the a solution, hc/hst was 1. Besides, Dong [71] et al. also prepared GQDs using CA as precursor, and the resultant size was betw and 8 nm as indicated by the TEM image of the N-GQDs sh Fig. 5d. The fringes of the carbon lattice can be clearly seen inset of Fig. 5d. Additionally, Chen and co-workers [77] fab amine-functionalized N-GQDs using CA and tris(hydroxym aminomethane (Tris-HMA), and its PLQY was 59.2%. CA as p sor has been widely adopted in the growth of GQDs using thermal method [78,79]. Apart from CA, some graphene-like smaller polycycl matic hydrocarbons (PAHs) can also be used as precursors preparation of GQDs. The GQDs emitting bright green fl Noviembre de 2020 24 cence with high yield of 63% were synthesized with pyre Fig. 4. The categorized illustrations on P. the Tianvarious et al.preparation methods / Materials Todayof GQDs. hydrothermal Chemistry 10 (2018) 221-258method by Wang et al. [72]. Fig. 5e depi 24 12
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Published on 28 June 2016. Downloaded by University
phite oxide and cited in their published bonded to aromatic carbon),75,82 C–O (oxygen singly
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74
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us simplied the GO synthesis process. This method for large-scale production of graphite oxide involves
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"1 88
(1580 cm ). The G-band, which is characteristic
troscopy (XPS),37,75–79 X-ray
olved needed to be removed by an inert gas, expanded graphite, which moderately oxidizes graphite. 36absorption
The oxygennear-edge
residual and other spectroscopy
hetero atoms, ashybridized
well as structural
carbon networks, originates from the
(XANES), 71,76,78,80,81
Fourier transform
ofdefects. Duringinfrared spectroscopy
the reduction processes, most oxygen-
s a constant hazard. Therefore, the further recent review by Wissler
Síntesis
is an excellent, succinct source further
p RSC Advances scattering from the doubly degenerate E2g phonon
velopment of the new process for oxidation (FTIR), 78,82–84
Raman spectroscopy containing functional
79,81,85 groups ofnuclear
and solid-state GO are eliminated and the
vestigation. This method and its modied graphite in the Brillouin zone center, while the pr
magnetic resonance.37,84,86,87 Mkhoyan
p-electron conjugation within the
et al.41 studied thearomatic
elec- system of graphite
en used by several other groups for the
is partially restored. Finally
peak comes from the with structural
some of imperfections crea
them bonded to oxyge
eldthe rGO gets of precipitated from the
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te oxide.55–60 tronic and atomic structure of GO using dark imaging of graphene with surface oxidation
reaction medium because of the recovered graphite domains of
single and multilayer sheets. The results of electron energy loss Functional groups attached on G
chemically converted graphene sheets with increased hydro-
thod: concentrated HNO3 acid, spectroscopy used for measuring the structure of carbon and mined by XPS and it reveals the nat
Fig. 5 Raman spectra of the GO and rGO powder. Reprinted 81 phobicity and p-stacking interaction. 95
The properties of rGO
4 acid and KClO3 oxygen K-edges in a scanning transmission electron microscope bonds in their various states as uno
(adapted) with permission from ref. 81. Copyright (2009) American are nearly 3
similar to that of graphene prepared through
et al.30,48 used concentrated sulfuric acid in indicate the high ratio of sp C–O bonds induces structural C–O, C]O, and COOH. Several XP
Chemical Society.
concentrated nitric acid and KClO3 for the different chemical, thermal, photo, electrochemical or micro-
distortions. This suggestswave that the atomicpathways.
structure 36 of GO sheets
conrms the peaks positions rela
te for the preparation of graphite oxide. The reduction The most widely applied technique
groups. The deconvoluted C1s sig
Published on 28 June 2016. Downloaded by University of California - San Diego on 22/04/2017 16:45:44.
strong oxidizing agent and it oxidizes the should resemble a mostlyused amorphous 2D sheet of carbon atoms
for preparing chemically converted reduced GO is the
acids solution and typically also is an in situ
composed of mainly ve peaks (at r
chemical reduction of GO as shown in Fig. 7.96 positions corresponding to sp2
, which acts as the reactive species. Several
ave synthesized GO for different application
Shin et al. used NaBH4 for the reduction of graphite oxide.
97
(284.5 eV) and C atoms bonded to
Fig. 1 Methods for synthesis of graphite oxide using graphite, acids
mann method.61,62 and oxidizing chemicals. The different molar concentrations of NaBH4 shows different epoxide (C–O–C, 286.55 eV), carb
order of
Fig.reduction conrmed
2 Representation by XRD
of the procedurespatterns used and to itformalsoGO showsstarting carboxyl groups (COOH, 289.2 eV).
enhanced
with electrical properties
graphite flakes. aer reduction.
Under-oxidized hydrophobic Fig. 8carbon showsmaterialthe of GO shows the diminishing of the
oyal Society of Chemistry 2016 RSC Adv., 2016, 6, 64993–65011 | 64995
recovered
XRD pattern during theoxide
of graphite purification of IGO, HGO,
aer reduction by NaBH and HGO+. The peaks and a very small fraction
4 which
increased efficiency of the IGO method is indicated by the very small
clearlyamount
shows of theunder-oxidized
shiing andmaterial broadening of peaks at different
produced.64 Reprinted (adapted)
1000 ! C.88 The other XPS analysis,
molarwithconcentrations of NaBH
permission from ref. 64. Copyright 4 . At higher NaBH
(2010) American 4 molar
Chemical lution of the C1s spectra using fo
Noviembre de 2020 25
concentrations
Society. (150 mM), the peak of the large interlayer C–OH, C–O–C, and COOH, while
Fig. 4 XPS spectra of GO and oxygen functionalities
distance disappeared RSC Adv.,
Fig. 3 Structure of GO.37 Reprinted by permission from Macmillan
and shied 2016, into6, 64993–65011
a broad peak near
different
iC]O groups. In similar way, th
2q ¼ 76 Reprinted
temperature. (adapted) with permissi
23.98.information
This 37),
implied onthat functional
commonly used groups
graphitewere removed.as(2011)
andCopyright
carbons, spectra contains the main peaks
well asAmerican
Publishers Ltd: [Nature chemistry] (ref. copyright (2009). 76. Chemical Society.
25 Fanthe al. reported
et terminology
98
used that the exfoliated
to describe graphite73 oxide can
these materials. 533.43 eV and these peaks are assi
Fig. 6 FTIR spectra of graphite oxide and NaOH–graphite oxide.78 bonded to aromatic carbon),75,82 C
Reprinted (adapted) with permission from ref. 78. Copyright (2010) undergo quick deoxygenation in strong alkali solutions like
NaOH3. aliphatic carbon), and phenolic
and KOH Functional
at moderate groups temperatures on (50–90graphene C) resulting This journal is ©
$
American Chemical Society. 64996 | RSC Adv., 2016, 6, 64993–65011 The Royal Society of Ch
in stable aqueous graphene suspensions. Fig. 9 shows that the aromatic carbon) 58,90
groups, respec
oxide and its analysis
addition of NaOH to the graphite oxide suspension led to the
an additional peak at a higher bind
attachment of oxygenated groups on the carbon basal plane. 92
The XANES is the other powerfu
change Thein structure
color (yellow-brown
of GO is described to dark asblack).
graphene sheet bonded with analysis of GO materials. It provide
Thus, the integrated intensity ratio of the D- and G-bands (ID/IG)
Various
severalinorganic
oxygen inand the organic reducing carboxyl,
form of hydroxyl, agents such epoxyasand degree of bond hybridization in m
indicates the oxidation degree and the size of sp2 ring clusters
phenylothers
hydrazine,
etc. groups99
hydrazine
as shown inhydrate, Fig. 3.37 95 Also, sodium
the structure borohy- of GO specic bonding congurations of
in a network of sp3 and sp2 bonded carbon.75,92 For example,
dride,97,100
depends ascorbic acid,101,102
signicantly on the glucose,
purication103
hydroxylamine,
procedures, rather 104
of alignment of the graphitic crysta
Mattevi et al., calculated the average graphitic domain size to
75
hydroquinone,
than, as is
105
pyrrole,
commonly 106
amino
thought, acids,
on the type
107,108
strongly
of graphite alkaline
used or Raman spectroscopy is an exp
be !2.5 nm in pristine GO. Lee et al.81 reported that aer
oxidation protocol.109 74 The exact identity and distribution of oxide
solutions and urea have been explored for the chemical
98
commonly used to characterize a
thermal reduction of GO, the intensity ration ID/IG signicantly
decreases and this indicates the considerable recovery of the
conjugated graphitic framework upon removal of epoxy and
4. Obtención de reduced GO (rGO)
functional groups depend strongly on the extent of oxidation. zero dimensions, such as 3D graph
The appearance of chemical composition inside GO and the nanotubes, and 0D fullerenes.2
oxygen containing functional groups in GO can be identied displays two major D band (1340 c
hydroxyl groups. Raman spectra in Fig. 5 clearly show that the using various techniques, including X-ray photoelectron spec- (1580 cm"1).88 The G-band, which View Article Online
intensity of G band of rGO isphighReducción Química
and sharp as compared toRSC
GO.Advances Review
troscopy (XPS),37,75–79 X-ray absorption near-edge spectroscopy hybridized carbon networks, orig
FTIR spectroscopy is an important tool for characterization (XANES), 71,76,78,80,81
Fourier transform infrared spectroscopy
as thermal reduction by annealing. Annealing atmosphere is
n NaBH4 also important for the reduction of GO. Annealing reduction is scattering from the doubly degen
of functional groups attached on GO surface. Fig. 6 shows the (FTIR),78,82–84 Raman spectroscopy usually carried out79,81,85 andorsolid-state
in vacuum, in inert nuclear graphite in the Brillouin zone cen
or reducing 110 111
p ofReducción
GO in FTIR and Térmica
atmosphere. Thermal reduction of GO comprised of the 111–113
position of absorption peaks it indicates that magnetic resonance. thermal-energy-induced
37,84,86,87
Mkhoyan et al.
multistep
41
removalstudied the
of intercalated H Oelec-
peak comes from the structural i 2
molecules and oxide groups of carboxyl, hydroxyl, and epoxy. It
different functional groups havendifferentexfoliación
bond energy.78 In the tronic and atomic structure should beof GOthatusing
noted dark
in chemical reduction, imaging
eldindividual GO of
sheets in the solution phase are chemically reduced by the
case of GO, it have different peaks as hydroxyl (broad peak at single and multilayer sheets. The results of electron energy loss
Published on 28 June 2016. Downloaded by University of California - San Diego on 22/04/2017 16:45:44.
p Reducción strong chemical base. The rapid heating of graphite oxide at 114
3050–3800 cm"1), carbonyl (1750–1850 cm"1), carboxyl (1650– spectroscopy used for measuring the structure of carbon and
high temperature, exfoliates in the form of porous carbon
1750 cm"1), C]C (1500–1600fotodegradacm"1), and etheryor epoxide
materials and get converted into graphene with fewer amounts
oxygen K-edges in a scanning transmission
of oxygen functionalities. electron
The exfoliation occurs bymicroscope
the sudden
expansion of3CO or CO gases evolved from the spaces between
(1000–1280 cm"1) groups.78,82,84microondas
2
indicate the high ratiographene of sp sheetsC–O bonds
during rapid induces
heating of structural
the graphite oxide. The
rapid heating makes the oxygen containing functional groups
distortions. This suggests thaton the
carbonatomic structure ofthat
GOcreate
sheets
p Reducción
attached plane to decompose into gases
should resemble a mostly amorphous 2D sheet of carbon atoms
huge pressure between the stacked carbon layers. Based on
state equation, a pressure of 40 MPa is generated at 300 C, !
Hidrotermal
Fig. 8 XRD patterns of GO, graphite (G), and rGO obtained using
4. Reduction of graphene oxide various molar concentrations of NaBH4.97 Reprinted (adapted) with
permission from ref. 97. Copyright © 2009 WILEY-VCH Verlag GmbH
while 130 MPa is generated at 1000 ! C.115 The evaluation of the
Hamaker constant predicts that a pressure of only 2.5 MPa is
& Co. KGaA, Weinheim.
Reducción Catalítica
enough to separate two stacked GO platelets.115 Rapid heat
4.1. Chemical reduction p treatment at elevated temperature, not only exfoliate graphite
oxide but also reduce the functionalities by decomposing
Chemical reduction of graphite oxide is one of the excellent Fig. 7
oxygen containing groups. The notable effect found during the
Preparation of chemically converted graphene (CCG) by
thermal exfoliation is the structural defect and damage of gra-
procedures to synthesized rGO and graphene in large quanti- 96 phene sheets caused by the release of carbon dioxide.90 These
reduction of GO. Reprinted (adapted) with permission from ref. 96.
structural defects and damage inevitably affect the electronic
ties.28,93,94 It includes ultrasonication of graphite oxide in water Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
properties by decreasing the ballistic transport path length and
introducing scattering centers.
The heating temperature severely affects the removal of
oxygen from GO surfaces and consequently effect the reduction
of GO.90,110,111 Li et al.112 monitored the variation of chemical
This journal is © The Royal Society of Chemistry 2016 structure with annealing temperature, and concludes that the
RSC Adv., 2016, 6, 64993–65011 |26 64997 Fig. 4 XPS spectra of GO and oxy
Noviembre de 2020
Fig. 3 Structure of GO.37highReprinted temperature is needed to achieve the better reduction of
by permission from Macmillan different temperature.76 Reprinted (ad
GO. Schniepp et al.90 reported the C/O ratio less than 7 for
Publishers Ltd: [Nature chemistry] (ref. less
heating temperature 37), copyright
than 500 ! C, and its(2009).
ratio increases to 76. Copyright (2011) American Chemi
higher than 13 as the heating temperature reaches to 750 ! C.
Fig. 9 (a) Illustration for the deoxygenation of exfoliated graphite The reduction of GO usually enhances the electrical conduc-
26 oxide under alkaline conditions and (b) images of the exfoliated- tivity due to the removal of oxygen containing groups. Wang
graphite oxide suspension ("0.5 mg mL#1) before and after reaction.
The control experiment in (b) carried out by heating the pristine 64996 | RSC Adv., 2016, 6, 64993–65011 This journal is © Th
exfoliated-graphite oxide suspension without NaOH and KOH at 90 ! C
for 5 h with the aid of sonication.98 Reprinted (adapted) with permis-
sion from ref. 98. Copyright © 2008 WILEY-VCH Verlag GmbH & Co.
KGaA, Weinheim.
reduction of GO. Wang et al. reported that high-temperature
alcohol vapor can also be used as an effective reducing agent
of GO and the resulting chemically converted reduced graphene
exhibits a highly graphitic structure and excellent electrical
conductivity.105
4.2. Thermal reduction
Thermal reduction of GO or graphite oxide is an important
13
Fig. 10Increase of the average conductivity of graphene films with
processing step in the synthesis of many graphene-based temperature.111 Reprinted (adapted) with permission from ref. 111.
materials and devices. The GO reduced by heating is known Copyright (2008) American Chemical Society.4. Obtención de GQD p Reducción de Tamaño por clivaje p Método de Sono Fenton n 50 mg de GO n 20 mL de H2O2 (30%) n 10 mg de FeCl3 (catalizador) n 100 mL de agua destilada n Ultrasonido por 4 hs a 90W (acelerador) n Diálisis por 3 días para remover restos de Fe. Article pubs.acs.org/JPCC Tuning Electrical Transport Mechanism of Polyaniline−Graphene Oxide Quantum Dots Nanocomposites for Potential Electronic Device Applications Noviembre de 2020 Dominique Mombrú, Mariano Romero,* Ricardo Faccio, and Á lvaro W. Mombrú* Centro NanoMat/CryssMat/Física, DETEMA, Facultad de Química, Universidad de la República, C.P. 11100 Montevideo, Uruguay J. Phys. Chem. C 2016, 120, 43, 25117–25123 27 ABSTRACT: In this report, we study the tuning of the electrical transport dimensionality of polyaniline−graphene oxide quantum dots nanocomposites (PANI-GOQD) for electronic device applications. We focused this study on the microstructure and its correlations with electrical transport properties. X-ray diffraction and small-angle X-ray scattering analyses showed the effect that caused the addition of GOQD on the structural and microstructural properties of polyaniline. Confocal Raman spectroscopy revealed that the presence of GOQD leads to a notorious decrease of the polaron population of polyaniline. In relation to this experimental evidence, a significant increase in resistivity was observed for PANI-GOQD nanocomposites with respect to pure polyaniline. Electrical transport showed a typical Arrhenius behavior at relatively high temperatures and a broad transition with a logarithmic dependence of the activation energy with temperature for the intermediate temperature regime. Additionally, PANI-GOQD showed an increase in the hopping transport dimensionality even in the case of low amounts of GOQD. The tuning of this dimensionality in these nanocomposites could be important for the development of novel organic 4. Caracterización de los GQD electronic materials. 1. INTRODUCTION effects of GOQD additions, due to their low dimensionality There is a recent interest in the preparation and character- with respect to graphene oxide (GO), could have consequences ization of conductive polymer based devices for electronic,1 in the electrical transport dimensionality of PANI-GOQD photoresponse,2 and energy conversion3 applications. Polyani- nanocomposites. Since small-angle X-ray scattering (SAXS) is line (PANI) has shown excellent processing and electrical usually performed to study the nanoparticles size, correlation properties among other conductive polymers for potential distances, and fractal dimensions; this technique is useful to electronic device applications.4−6 There are recent reports on correlate the last one with electrical transport dimensionality. the preparation and electrical transport properties of PANI Additionally, there are very few experimental correlations nanocomposites with additions of different materials such as between the polaron populations and the electrical transport oxide nanoparticles7,8 or carbon-based nanostructures.9,10 properties in these conductive polymer nanocomposites.15 These experimental and theoretical reports study the electrical Raman spectroscopy, which is another powerful tool to transport mechanism of these polyaniline based materials. The characterize the formation of polarons in polyaniline, could preparation and electrical characterization of polyaniline− be used as a key technique for that purpose. graphene oxide quantum dots (PANI-GOQD) nanocomposites This report is about the preparation and microstructure has been recently reported as promising materials for characterization of PANI-GOQD nanocomposites by means of optical11,12 and supercapacitor electrode13,14 applications. X-ray powder diffraction, small-angle X-ray scattering, and However, studies regarding the electrical transport mechanism confocal Raman spectroscopy. Here we correlate micro- and charge carrier dimensionality in these PANI-GOQD structural characterization with the electrical transport perform- nanocomposites are still to be performed. Although it is accepted that the electrical transport mechanism is based Received: September 5, 2016 mainly on the hopping process of charge carriers,5 the possible Revised: October 12, 2016 correlation with microstructure remains open. In fact, the Published: October 12, 2016 © 2016 American Chemical Society 25117 DOI: 10.1021/acs.jpcc.6b08954 J. Phys. Chem. C 2016, 120, 25117−25123 Contacto AC Mode Noviembre de 2020 28 28 14
4. AFM de GO Noviembre de 2020 29 29 4. AFM de GOQD Noviembre de 2020 30 30 15
4. AFM de GOQD p 60 nm de diámetro y 13 nm de alto Noviembre de 2020 31 31 The Journal of Physical Chemistry C Article π−π stacking interaction leading to an enhancement on the degree of order in the polymer structure.22 Differential scanning calorimetry (DSC) analysis for PANI- GOQD-X is shown in Figure 3b. The glass transition temperature (Tg) estimated from the DSC curves were Tg ≈ 100 and 128 °C for X = 0 and 1, respectively. Moreover, X = 3 and 5 showed a well-defined endothermic peak associated with the melting process (Tf) of crystalline zones in PANI-GOQD-X nanocomposites at Tf ≈ 160 and 172 °C, respectively. In both cases, the increase of both Tg and Tf with increasing X is 4. Raman de GOQD suggesting an increase in the degree of order of polyaniline fibers for higher amounts of GOQD additions. The increase in the degree of order with increasing X is also consistent with the appearance of the new sharp peak at 2θ ≈ 19° in the XRD patterns, observed for higher amounts of GOQD additions. 60 nm de diámetro y 13 nm de alto Raman spectra for PANI-GOQD-X are shown in Figure 4a, The Journal of Physicalpand Chemistry both the Cvibrational modes frequencies and assignments Article are summarized in Table 2. Raman spectra for pure GOQD showed two typical broad peaks at ∼1344 and 1599 cm−1, ascribed to D and G modes, respectively. Both the frequency and The width Journal of peaks are in agreement of Physical Chemistry withC GOQD with a 20−30 Article nm diameter size according to a systematic study already 23 reported. However, Raman spectra π−π stacking interaction leading to an enhancement on the for pure PANI showed degree atof ∼1167 peaks order inand the 1260 polymer cm−1 ascribed structure. 22 to C−H bending modes of the aromatic Differential scanning ring, at ∼1334 calorimetry cm−1 analysis (DSC) to the polaron for PANI- C− −1 N +• GOQD-X stretching mode, atin∼1415 is shown Figureand3b.1486 Thecmglass to C−Ntransition and −1 CN stretching modes, and at ∼1580 temperature (Tg) estimated from the DSC curves were Tg ≈ and 1636 cm ascribed 24,25 to C−C 100 and 128 and °C CC for Xstretching = 0 and 1, mode of the aromatic respectively. Moreover, ring.X=3 +• However, and 5 showed the peak ascribed to a well-defined the polaron endothermic peak C−N stretching associated with mode the for PANI-GOQD-X melting process (Tf) of showed crystalline a remarkable zones in PANI-GOQD-X decrease in its relative intensity at nanocomposites with ≈ 160 and Tf GOQD 172 °C,asrespectively. additions, shown in Figure In both 4b. This cases, observation the increaseis ofsuggesting both Tg that and T thef GOQD with addition increasing X is somehow interacting suggesting an increasewith in the the degree charge ofcarriersorder of polyaniline leadingfortohigher fibers a suppression amounts ofofGOQD the polaron additions. population. The increase PANI- in the degree of order with increasing X is alsoincrease GOQD-X with X = 3 and 5 showed an consistent of with GOQD the peaks at ∼1344 appearance of the andnew 1599 cm−1peak sharp , as aatmere2θ ≈consequence 19° in the of XRD its increasingobserved patterns, concentration. for higher Nevertheless, amounts the of GOQDpeak at ∼1167 additions. cm−1 −1 showed Ramana spectra shift toforhigher frequenciesareat shown PANI-GOQD-X ∼1186in cm Figurewith 4a, Article GOQD and bothadditions, probablymodes the vibrational associated with theand frequencies transition assignments of the pubs.acs.org/JPCC polaron are to the bipolaron summarized in Table 2. conformation, Raman spectra as itforwas pure previously GOQD Tuning Electrical Transport Mechanism of Polyaniline−Graphene reported two showed in our previous typical broad study peaks using ∼1344 functional at density and 1599 theory cm−1, Oxide Quantum Dots Nanocomposites for Potential Electronic Figure 1. AFM images for (a) GOQD samples and (b) 15 PANI-GOQD-X nanocomposites with X = 0, 1, 3, and 5. Topography analysis was obtained Noviembre (DFT) ascribed from the cross-section marked delines 2020 withsimulations. to D and in the G modes, images. respectively. GOQD histogram shown in (a) Both was the frequency obtained Device Applications from a large representative cross-section. 32 Dominique Mombrú, Mariano Romero,* Ricardo Faccio, and Á lvaro W. Mombrú* andImpedance width of peaks spectroscopy analysis for are in agreement withPANI-GOQD-X GOQD with a 20−30 nano- ance in order to reveal the composites nm transport diameter mechanism is shown size in theseto in Figure according a compressed 5. Phase (ϕ) versus systematic at 50 kNfrequency study foralready 10 min to formFigure pellets with 4. a(a) ∼800 Centro NanoMat/CryssMat/Física, DETEMA, Facultad de Química, Universidad de la República, C.P. 11100 Montevideo, Uruguay Raman spectra and Lorentzian deconvolution for conductive polymer nanocomposite plots showed reported. 23 systems. zero values However, Raman wide μm for aspectra rangethickness and 12 mm diameter. for pure 2.3. of frequencies PANI showed Characterization. (f = GOQD and PANI-GOQD-X with X= 0, 1, 3, and 5. (b) Raman PANI-GOQD-X nanocomposites 2. 32 −2 MATERIALS ANDpeaks −10 10METHODS 5 at ∼1167 Hz) suggesting and 1260a resistivecm−1 ascribed behavior. were studiedto byThe X-raypositive C−H bending diffraction ϕ(XRD) using relative intensity a Rigaku Ultimaand shift of frequencies for main selected peaks. −1 5 values observed modes 2.1. Preparation of GOQD. ofGraphene at higher the aromatic ring, oxide quantum dots at ∼1334IV( cm frequencies f diffractometer > 10toHz) the are probably polaron working inC− Bragg−Brentano configuration −1 N +• attributed stretching to a small mode, were prepared using graphene oxide (GO) as precursor using at ∼1415 inductance behavior and with 1486 CuKα associated cm radiation to inwith C−N the 2θand the = 5−50° range using 2θ steps of analyzed. This is also in agreement with the decrease of polaron 0.02°modulus with a 5 s also integration −1 showed time per step. Grazing incidence the sonoFenton method. device CN 16 wires GOstretching precursor 17 contribution. modes, was prepared at ∼1580 Impedance andusing and 1636 cm small-angle X-ray scattering24,25 ascribed (SAXS) was population performed with ABSTRACT: using increasing In this report, we study the GOQD additions, tuning of the electrical envisaged transport dimensionality byoxidethe of polyaniline−graphene the Hummer’s modified constant to method. C−C and impedance GOQDCC values preparation stretchingin a mode starts broadRigakufrequency of the regime aromatic Ultima IV withsystem ring. diffraction Z working Raman analysis. in quantum dots nanocomposites (PANI-GOQD) for electronic device applications. We focused this study on the microstructure parallel and beam with its correlations The electricalhigher temperature transport properties. regime X-ray diffraction and small-angle (∼300−255 X-ray scattering analyses showed the with 20 mL of H2O2 (30%)≈ 8, 10,and 30, 10 mg and of 90 FeClohm·cm 3 added tofor 50 X = 0, 1, 3, and 5, +• withrespectively. effect that caused the addition of GOQD on the structural and microstructural properties of polyaniline. Confocal Raman However, mg of GO precursor dispersion the in 100 mLpeak ascribed of distillated to water, the polaron configuration C−N stretching CuKα radiation in theK)q =can 0.01−0.5 relationdescribed −1 spectroscopyÅrevealed that the presence of GOQD leads to a notorious decrease of the polaron population of polyaniline. In be using to this experimental evidence, the a significant increaseArrhenius in resistivity was observedlaw, which for PANI-GOQD canwithbe nanocomposites under stirring at room The increase modetemperature. inThe for PANI-GOQD-X the solution impedance was modulus showed a range andwas remarkable fixed observed incident decrease angleinwith atits0.2° with respect to the respectcritical expressedtransition to pure polyaniline. Electrical transport showed a typical Arrhenius behavior at relatively high temperatures and a broad by withthe following a logarithmic dependence of the equation: activation energy with temperature for the intermediate temperature regime. submitted to ultrasonicincreasing GOQD aadditions, powerinof angle. Differential agreement with theinscanning decrease calorimetry in (DSC) was performed relative intensity with GOQD additions, asShimadzu shown Figure 4b. is temperature range T = 25−⎛ Ea,0 ⎞ Additionally, PANI-GOQD showed an increase in the hopping transport dimensionality even in the case of low amounts of treatment working at fixed GOQD. The tuning of this dimensionality in these nanocomposites could be important for the development of novel organic using DSC-60 differential scanning calorimeter electronicusing materials. exp⎜ − ⎟ T > T1 ≈effects255 90 W during 4 h. The the graphene This oxide quantum polaron observation is dots population (GOQD) envisaged suggesting by the our thatnitrogen GOQD Raman analysis addition 50 mL/min flow at the σ(T )1. ≈ K additions, due to their low dimensionality There is a recent⎝ ⎠ and character- with solution was finally dialyzed for 3 days in order to remove discussed somehow interactingabove. In all withcases, a moderate the charge 200 °C withdecrease carriers rate in of polyaniline the Atomic force INTRODUCTION interestkin the of GOQD BTpreparation residual iron. a ramp of 5°/min. microscopy respect to graphene oxide (GO), could have consequences 2.2. Preparation impedance to awas observed of for the the polaron low frequency regime, as Raman spectroscopy in the electrical transport dimensionality of PANI-GOQD 16 leading suppression (AFM) inpopulation. the AC mode PANI- and confocal was polymer based devices for electronic, nanocomposites. 1 ization of conductive of PANI-GOQD-X Nanocomposites. 2 photoresponse, and energy conversion applications. Polyani- 3 Since small-angle X-ray scattering (SAXS) is performed 26 using WITec Alpha 300-RA. AFM and Raman data usually performed to study the nanoparticles size, correlation GOQD aqueous solution already GOQD-X observed was dried 100 °C with at Xfor = other for polyaniline 3 andhours several 5 showed composites. an increase of GOQD for which line σ device (PANI) properties isamong the has other dc shown conductivity, excellent conductive polymers for potential Edistances, processing and electrical a,0 isthe the correlate activation and fractal dimensions; energy, this technique is useful to last one with electrical transport dimensionality. −1 acquisition for GOQD sample was obtained by placing electronic a applications. 4−6 There are recent reports on to remove residual waterpeaks at ∼1344 and Resistivity resuspended andin1599 versus 10 mL cm of , ascurves temperature merefor adroplet PANI-GOQD-X ofconsequence the GOQD suspension of its on a and kB nanocomposites the is silicon substrate the preparation and Boltzmann and electrical transport properties ofconstant. with additions of different materials such as PANI between theThe Additionally, there linearization are very few experimental correlations polaron populations of and the electrical transport tetrahydrofuran with different amounts of GOQD additions. −1 −1 nanocomposites. 15 nanocomposites are shown in Figure the 6. ∼1167 A attypical semi- cmmin. RamanArrhenius equation represented by characterize properties in these conductive polymer the spectroscopy, ln(σ) vs isTanother plots powerful toolare 7,8 9,10 PANI emeraldine salt increasing concentration. Nevertheless, peak oxide nanoparticles or carbon-based nanostructures. (PANI-DBSA-H SO ) with average 2 4 Mw drying at ∼180 °C for 15 spectra These for experimental PANI-and theoretical reports study the electrical Raman which the formation of polarons in polyaniline, could to −1 transport mechanism of these polyaniline based materials. The
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