Photoresponse of Physically Oxidized Graphene Sensitized by an

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Photoresponse of Physically Oxidized Graphene Sensitized by an Organic Dye Jaehun Han, Youngbin Lee, Samudrala Appalakondaiah, Jinshu Li, Xing Gao, Youngjae Yoo, Dongmok Whang, Euyheon Hwang, and Jeong Ho Cho J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b00712 • Publication Date (Web): 21 Mar 2017 Downloaded from http://pubs.acs.org on March 28, 2017

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The Journal of Physical Chemistry

Photoresponse of Physically Oxidized Graphene Sensitized by an Organic Dye Jae Hoon Han,1 Youngbin Lee,1 S. Appalakondaiah,1 Jinshu Li,1 Xing Gao,1 Youngjae Yoo,4 Dongmok Whang,1 Euyheon Hwang,1,2* and Jeong Ho Cho1,3* 1

SKKU Advanced Institute of Nanotechnology (SAINT), 2Department of Physics, 3School of Chemical Engineering, Sungkyunkwan University, Suwon, 440−746, Korea. 4 Division of Advanced Materials, Korea Research Institute of Chemical Technology, Daejeon 305-600, Korea. *Corresponding author: J. H. Cho (email: [email protected]) and E. Hwang (email: [email protected])

Abstract We investigated the charge transport and photoresponse characteristics of a hybrid structure comprising physically oxidized graphene and Rhodamine-based organic dye molecules. The oxidation of the graphene surface was deterministically controlled by varying the UV/ozone exposure time. The oxidized graphene surface was then modified with the organic dye molecules using a simple dip-coating method. The electrical conductance and photoresponse of the resulting hybrid films were investigated systematically using Raman spectroscopy and environment-dependent charge transport measurements. The oxygencontaining groups generated by the UV/ozone exposure dramatically enhanced the photoresponses of the hybrid films while maintaining a high device performance. Importantly, we found that the photoresponses of the hybrid films were strongly related to chemical reactions between the photoexcited electrons and adsorbates (water or oxygen) in the dye layer as well as to the migration of the photoexcited electrons toward the top surface of the dye layer due to the negatively charged oxygen-containing groups at the graphene–dye interface. Our simple and effective method involving oxidizing graphene and hybridizing the resultant layer with organic dyes will inspire new approaches to the development of electronics and optoelectronics based on graphene.

Keywords: graphene, UV/ozone oxidation, organic dye, photoresponse, charge transport

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1. INTRODUCTION Graphene, a two-dimensional sp2-hybridized single layer of carbon atoms, has great potential in numerous applications related to light detection owing to its high carrier mobility, gapless spectrum, and frequency-independent absorption1-7; however, the intrinsically low light absorption of graphene (2.3% saturable absorption), low gain mechanism, and fast recombination rate have limited the application of the material in practical high-performance optoelectronic devices.8-10 Pristine graphene-based photodetectors exhibit an internal quantum efficiency of 30% and an external responsivity of 10 mA/W, much lower than the corresponding values obtained from commercial devices.11-13 The absorption of light by graphene has been enhanced by decorating graphene surfaces with a variety of light-harvesting materials, including gold nanoparticle-based

plasmonic

resonators,13-15

perovskites,19 and 2D semiconductors.20,

21

colloidal

quantum

dots,16-18

organometallic

halide

Among these materials, organic dye molecules with a high

absorption cross-section have been recognized as particularly effective light-harvesting materials.22-24 Commercially available dye molecules with a variety of absorption spectra have been widely used to enhance the photoresponses of graphene. The solution processability of an organic dye permits deterministic control over the deposition density through variations in the dye concentration or deposition time. Strong π– π interactions between the conjugated organic dyes and graphene facilitate illumination-induced charge transfer between the two layers, thereby enhancing the photocurrent gain.25-28 Recently, hybrid structures comprising organic dye molecules (e.g., Rhodamine 6G (R6G)) and graphene have been demonstrated, and the resulting photodetectors have provided very large photocurrents and ultrahigh quantum efficiencies with spectral color selectivity.29, 30 The oxidation of graphene has been widely studied as a means for modifying the properties of graphene.31-34 The poor electrical conductivities of graphene oxide (GO) or reduced form of GO (rGO) due to severe structural defects limit their applicability to high-performance electronic devices.35, 36 Recently, physically oxidized graphene has attracted interest because unlike GO or rGO, it is electrically conductive.3739

In addition, physically oxidized graphene substrates give rise to surface-enhanced Raman scattering with a

large chemical enhancement over large scales.40 Thus, oxidized graphene may be used in a range of applications, including electronic and optoelectronic technologies. Hybridization with organic dyes having a high absorption cross-section can enable new high-performance optoelectronic device applications. The influence of chemically functionalizing graphene surfaces on the illumination-induced charge transfer between the graphene and organic dye layers has not previously been described. Such studies have not been conducted even on other hybrid structures. Furthermore, the effects of environmental factors on the electrical properties must be examined to understand the photoresponses of graphene–organic dye hybrid structures. In this manuscript, we studied the transport and photoresponse properties of a physically oxidized graphene layer decorated with Rhodamine-based organic dye molecules. The graphene was physically 2


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oxidized via simple UV/ozone exposure of the graphene surface over a short period of time. The chemical and electrical properties of the graphene after UV/ozone exposure were systematically investigated using xray photoemission spectroscopy and charge transport measurements. We found that the oxygen-containing groups on the graphene surface widened the minimum conductance region, generating asymmetric conductance between the electron and hole sides, and increased the hysteresis window. The oxidized graphene was hybridized to the organic dye molecules using a simple dip-coating method. The photoresponse of this hybrid film was strongly related to chemical reactions between the photoexcited electrons and adsorbates (water or oxygen) in the dye layer as well as to the migration of the photoexcited electrons to the top surface of the dye layer due to the presence of UV/ozone-induced negatively charged impurities at the graphene–dye interface. The simple physical oxidation of the graphene surface and its hybridization with organic dye molecules provides a clever approach to enhancing the photoresponse characteristics of graphene-based optoelectronic devices.

2. RESULTS AND DISCUSSION Figure 1 shows a schematic illustration of the optoelectronic devices developed here based on a UV/ozone-oxidized graphene channel layer hybridized to the Rhodamine 6G (R6G) organic dye. Transistors were fabricated on heavily doped Si substrates with a 70 nm thick Al2O3 gate dielectric layer deposited by atomic layer deposition (ALD). Au source–drain electrodes were deposited thermally onto the Al2O3 surface through a metallic shadow mask. The channel length and width were 100 and 1000 µm respectively. The chemical vapor deposition (CVD)-grown single-layer graphene was transferred onto the substrate and then patterned to form an active channel via conventional photolithography and subsequent reactive ion etching (RIE). The graphene channel was oxidized physically by UV/ozone exposure, and the degree of oxidization was controlled by varying the exposure time (0, 15, or 30 s). A low-power mercury lamp with wavelengths of 254 nm (80%) and 184 nm (20%) was utilized for the UV/ozone treatment. Finally, R6G organic dye molecules were deposited onto the graphene channel surface via a dip-coating method. The UV-vis absorption spectrum of the R6G organic dye coated onto the graphene layer is shown in Figure S1. The spectrum exhibited an absorption peak at 530 nm, which corresponded to the direct transition from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) of R6G.41, 42

The effects of UV/ozone oxidation of the graphene were investigated using x-ray photoemission spectroscopy (XPS) and charge transport measurements. First, XPS analysis was performed to characterize the changes in the chemical functional groups of graphene upon UV/ozone exposure. Figure 2a shows the XPS C1s spectra of pristine graphene and graphene oxidized by UV/ozone exposure for 15 or 30 s. The C1s spectra were deconvoluted into four distinct components, including sp2-hybridized carbon at 284.5 eV, epoxide (C-O-C) at 285.5 eV, hydroxyl (C-OH) at 286.9 eV, and carboxyl (COOH) at 288.9 eV, in good 3



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agreement with previous reports.40 The relative quantities of oxygenated carbon atoms present in epoxide, hydroxyl, and carboxyl groups increased gradually with increasing UV/ozone exposure time from 0 to 30 s, as shown in Figure 2b. Exposure to UV light with a wavelength of 185 nm transformed the oxygen (O2) molecules in ambient air into ozone (O3), and the generated O3 could be decomposed into oxygen ions (O–) under UV light of wavelength 254 nm. The resulting O– species attacked the graphene basal plane to form oxygen-containing groups, including epoxide, hydroxyl, and carboxyl moieties.43, 44 The effects of the generated oxygen-containing groups in graphene on the charge transport behavior were investigated using top-contact and bottom-gate transistors. Figures 2c shows the representative transfer characteristics (drain current (ID) vs. gate voltage (VG) plots) of the graphene transistors under dark conditions before and after UV/ozone exposure of the graphene channel at a fixed drain voltage (VD) of 0.1 V. The curves exhibited typical V-shaped I-V relations because the ambipolar graphene layer was capable of both electron and hole transport. The Dirac voltage (VDirac; charge neutral point of graphene) of the pristine graphene was –3.8 V. This value shifted gradually toward positive gate voltages to –0.7 V as the UV/ozone exposure time increased from 0 to 30 s (Figure 2d), indicating that more hole doping occurred as the UV/ozone exposure time increased. The hole doping induced by the oxygen-containing groups was also confirmed by the density functional theory calculations (Table S1). The changes in the carrier mobility and conductance were monitored as a function of the UV/ozone exposure time. The hole and electron mobilities of the pristine graphene were estimated to be 1054 and 442 cm2/Vs, respectively, and these values decreased gradually with the UV/exposure time: 844 cm2/Vs for holes and 265 cm2/Vs for electrons after a 30 s UV/ozone exposure. The reduction in the electron mobility exceeded that in the hole mobility. This asymmetry was also observed in the conductance values at various UV/ozone exposure times. Three interesting salient features were observed in the transfer curves at different UV/ozone exposure times: (i) the minimum conductance region broadened, (ii) the conductance between the electron and hole sides became asymmetric, and (iii) the hysteresis window broadened. These behaviors could be explained in terms of scattering off the negatively-charged oxygen-containing groups.45, 46 These groups are most likely to contribute to scattering mechanisms that limited graphene conductance. The long-range effects of the charged impurities affected both the ground state density profile and the transport properties of graphene. The broadening of the minimum conductance at the Dirac point was closely related to the charged impurity density.47, 48 The width of the minimum conductance was approximately proportional to the density of charged impurities. Density fluctuations arising from the charged impurities near the Dirac point broke the system into puddles of electrons and holes. The break-up of the graphene landscape into inhomogeneous puddles of electrons and holes gave rise to a finite conductance near the Dirac point. Thus, the width of the minimum conductance at the Dirac point increased with the UV/ozone exposure time because the number of negatively charged oxygen-containing impurities increased. The asymmetry in the observed conductance could also be explained in terms of the adsorbed impurities through the charge redistribution. Changing the 4


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sign of the bias voltage did not affect the concentration of charged impurities, but it changed the average distance between 2D graphene sheets and the charge centers of impurities because of the spatial redistribution of charges. The oxygen-containing impurities were negatively charged, and they responded differently to the sign of the applied gate voltage. The resistance was proportional to where kF is the Fermi wave vector and is given by = √ , with n corresponding to the carrier density and d the distance between 2D graphene and impurities; therefore, the conductance was easily affected by the locations of the impurities.49 A positive gate voltage shifted the charge distribution of negatively charged impurities closer to the graphene layer and reduced the conductance and mobility. A small shift of d = 1–2 Å was sufficient to explain the asymmetric behavior observed in experiments. Hysteresis (i.e., differences in the VDirac values obtained from the forward and reverse VG sweeps) increased gradually with increasing UV/ozone treatment times, as shown in Figure 2d. The charged impurities also contributed significantly to the current hysteresis.50-52 Charge transfer between the impurities and graphene contributed to the hysteresis. Under a large negative gate voltage, holes in graphene became trapped at negatively charged impurities, so that the graphene experienced a more positive potential than the potential due simply to the gate voltage (and vice versa for the opposite sweep direction). The initial trapped charge was proportional to the trapped centers; thus, as the UV/ozone exposure time increased, the hysteresis increased, as observed in Figure 2d. The illumination-induced chemical mechanism underlying the behavior of the hybrid film was investigated by monitoring the Raman signal of R6G adsorbed onto the graphene surface. Raman signals were collected using a laser with the resonant excitation wavelength (535 nm) of R6G to maximize the resonant Raman enhancement effect. Figure S2 shows the Raman spectrum of an R6G film deposited onto a Si wafer. The spectrum was consistent with an overwhelming fluorescent (FL) background because the cross-section of FL far exceeded that of resonance Raman scattering. All R6G samples coated onto graphene surfaces, however, provided R6G Raman band intensities with a high signal-to-noise ratio, as shown in Figure 2e. We understood that the enhanced Raman signal intensity arose from FL quenching in the dye molecules. Photo illumination reduced the rate of photoexcited electron–hole pair recombination, which gave suppressed the FL intensity.53, 54 As explained below, photoexcited electrons reacted with adsorbates (e.g., water and oxygen) in the vicinity of the graphene–dye interface. Electrons in graphene could be transferred to the HOMO level of the dye before the photoexcited electrons recombined with holes in the dye layer. The Raman signal increased dramatically with the UV/ozone exposure time because the enhancement was strongly correlated with the rate of chemical reactions between the photoexcited electrons and adsorbates, as well as with the rate of charge transfer between the two layers. Below, we explore why the chemical mechanism that enhanced the Raman signal also enhanced the photoresponses of the graphene and organic dye molecule hybrid structures. We investigated the photoresponse behaviors of the oxidized graphene–R6G organic dye hybrid 5



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films. Figure 3a presents the total drain current (dark current plus photocurrent) of transistors based on pristine graphene–R6G hybrid films as a function of the gate voltage under various illumination conditions. The measurements were conducted under ambient conditions, and the illumination wavelength was fixed to 520 nm. The VDirac shifted gradually toward a positive gate voltage as the optical power increased. Interestingly, the VDirac shift at each optical power increased dramatically after UV/ozone oxidation of the graphene channel, as shown in Figures 3b and 3c. For example, the pristine graphene device exhibited a VDirac shift of 0.3 V (after 100 µW illumination), whereas the values obtained from the oxidized graphene devices were 0.6 and 1.2 V for oxidation over 15 and 30 s, respectively. The VDirac shifts for all samples are summarized as a function of the optical power in Figure 3d. Figure 3e plots the photocurrents (Iph = Iillumination – Idark) in the devices measured at various VDirac values under dark conditions. These results are consistent with the measured optical power-dependent VDirac shifts. An important figure of merit for evaluating a photodetector’s performance is the responsivity (R), which is defined as Iph/P, where Iph is the photocurrent and P is the illumination power. The value of R in the hybrid films was inversely proportional to the illumination power (Figure 3f). The maximum R at an optical power of 1 µW was found to be 50 A/W for the oxidized graphene–R6G (for 30 s) hybrid films. Assuming that the relationship R ∝ P–1 holds, R is expected to exceed 105 A/W at an illumination power of 1 pW. The photoresponse behaviors of the oxidized graphene–organic dye hybrid films were explored by measuring the photoinduced transfer curves under two environmental conditions: ambient air or vacuum conditions (10–4 Torr). Figure 4a plots the total drain currents of the transistors based on the pristine graphene–R6G hybrid films as a function of the gate voltage at various optical powers. Surprisingly, under vacuum conditions (right panel of Figure 4a) VDirac shifted toward negative voltages as the optical power and the hole current increased, the reverse of what was observed under ambient conditions (left panel of Figure 4a). Opposing shifts in VDirac under ambient and vacuum conditions were also observed for the oxidized graphene–organic dye films, as shown in Figure 4b and Figure S3. Note that the photoinduced VDirac shift increased with the UV/ozone exposure time, as summarized in Figure 4c. The left-shift or right-shift in the Dirac point measured here under illumination was related to the doping of adsorbates (e.g., water (H2O) and oxygen (O2)) in the organic dye layer. In ambient air, water and oxygen molecules readily adsorbed onto the organic dye surface and then penetrated the dye layer. The adsorbates in the dye layer readily captured photoinduced electrons in the dye layer through the chemical reaction H2O(g) + 2e– → H2(g) + O2– (ad) and O2(g) + 4e– → 2O2– (ad).55 Therefore, capacitive gating (or photogating) occurred when the ionized oxygen species (O2–) generated by the reaction with adsorbates altered the local electrostatic potential around the graphene, which pulled more holes onto the graphene channel from the contacts and shifted the Dirac point toward positive gate voltages (see the upper panel in Figure 4d). Under vacuum, no unintentional adsorbates (water or oxygen) were present at the graphene–dye 6


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interface. When the electron–hole pairs were produced under light illumination, the photoexcited electrons moved away to the upper surface of the organic dye layer due to the presence of negatively charged oxygencontaining groups at the graphene–dye interface, resulting in more holes (than electrons) near the graphene– dye interface. Therefore, the dye layer acted as an effectively positive gate, which shifted the Dirac point toward negative gate voltages (see the lower panel in Figure 4d). In our devices, the direction of the Dirac voltage shift was related to the chemical reactions between the photoexcited charges and adsorbates as well as the migration of photoexcited charges to the top surface of the dye layer. The two mechanisms generally coexist and compete with one another; however, the carrier type induced in the graphene channel was opposite and shifted the Dirac voltage with respect to the gate voltage in the opposite direction. Importantly, the Dirac voltage shift increased with the number of oxygencontaining groups produced during UV/ozone exposure (see Figure 3). Under ambient conditions, a greater number of oxygen-containing groups on the graphene surface captured more adsorbates at the graphene–dye interface, which intensified the chemical reaction between the photoexcited electrons and adsorbates near the interface. As a result, capacitive gating became stronger. The movement of more holes toward graphene from the contacts was accompanied by a positive shift in the Dirac point. However, the absence of adsorbates at the interface (under vacuum conditions) resulted in the movement of more photoexcited electrons toward the top of the dye layer due to the presence of a greater number of negatively charged oxygen-containing groups at the interface, which retained holes within the organic dye layer. A stronger effective positive gating in the dye layer increased the magnitude of the negative shift in the Dirac point in graphene. Illumination-induced control over VDirac in the graphene transistors enabled the fabrication of logic inverters with controlled signal inversion, as illustrated schematically in Figure 5a. Here, the R6G-oxidized graphene (for 30 s) channel layer of one transistor was encapsulated using ALD-grown Al2O3 to minimize the effects of adsorbates (water or oxygen) on the channel. Al2O3 passivation effectively protected the graphene surface from interactions with ambient species.56, 57 The transistor 1 exposed to ambient air was connected to the supply electrode, whereas transistor 2 was connected to ground. The two transistors shared the same input (VIN) and output terminals (VOUT). The corresponding equivalent circuit diagram is shown in the right panel of Figure 5a. Figure 5b plots the voltage transfer characteristics of the logic inverters upon light illumination (wavelength = 520 nm) onto transistor 1. As the optical power increased, the signal inversion of the device shifted positively. The shift in the inverter’s signal inversion agreed well with the illumination-induced shift in the transfer curves (see the left panel of Figure 4b) because it occurred at a voltage that yielded the same resistance value for two graphene channels of the circuit. By contrast, the signal inversion of the logic inverter shifted toward negative voltages as the optical power increased during illumination of the encapsulated graphene channel on transistor 2 (Figure 5c). This trend was consistent with the negative shift in the transfer curves measured under vacuum conditions (see the right panel of 7



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Figure 4b). Although the separation between the logical levels (the difference between the ON and OFF states of the inverter) was relatively small (0.3 V in this work), modulation of the inversion point between the ON and OFF states could be precisely controlled via illumination-induced doping.

3. CONCLUSIONS In conclusion, we studied a hybrid structure formed using physically oxidized graphene and organic dye molecules. The transport properties and photoresponses of the resulting hybrid films were systematically investigated using Raman spectroscopy and environment-dependent charge transport measurements. The photoresponse properties of the hybrid film were governed mainly by chemical reactions between the photoexcited charges and adsorbates as well as the migration of the photoexcited charge toward the top surface of the dye layer. Our simple and effective graphene oxidation and its hybridization with organic dyes will inspire new approaches to the development of graphene-based electronics and optoelectronics. This hybrid structure provides a promising two-dimensional nanomaterial platform.

4. METHODS Device fabrication: high-quality single-layer graphene films were synthesized using the previously reported thermal chemical vapor deposition (CVD) method on a folded Cu foil (Alpha Aesar). Poly(methyl methacrylate) (PMMA, 46 mg/mL) dissolved in chlorobenzene was spin-coated onto the graphene film on copper (4000 rpm, 30 s). The graphene layer grown on the back side of the Cu film was removed by reactive ion etching (RIE). A 0.2 M aqueous solution of ammonium persulfate was used to remove the Cu foil. The graphene/PMMA layer was transferred onto a 70 nm thick Al2O3/heavily-doped Si substrate with prepatterned source and drain electrodes (Cr/Au = 3/30 nm). The channel width and length were 1000 and 100 µm, respectively. The PMMA supporting layer was removed using hot acetone (60°C, over 30 min). The graphene channels were patterned by photolithography and RIE using O2 plasma. The graphene transistors were immersed in a 100 µM aqueous solution of R6G organic dye (Aldrich Co.) for 30 min. The samples were then rinsed with copious amounts of deionized water for 10 min and dried under a N2 flow. Measurements: The chemical functional groups present on the graphene films were analyzed using XPS (Kalpha, Thermo Fisher). Raman measurements (excitation wavelength = 532 nm and power = 100 µW) were carried out using a commercial confocal Raman system (Witec Alpha 300 M+). The photoresponse characteristics of the devices were measured using an Agilent 4155 semiconductor parameter analyzer under ambient and vacuum conditions. A monochromatic laser (Susemicon) with a wavelength of 520 nm and a power of 4 µW was used as the light source (beam radius of 3 µm). The illumination power was controlled using an optical attenuator (Thorlabs NDC-50C-4M) and was measured using a laser power meter (Thorlabs PM 100D). 8


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Supporting information The Supporting Information is available free of charge on the ACS Publications website at DOI: UV-vis absorption spectrum and Raman spectrum of the R6G films, computational details, R6G film, illumination-dependent transfer characteristics of the graphene-organic dye hybrid transistors.

Acknowledgement This work was supported by Samsung Research Funding Center of Samsung Electronics under Project Number SRFCMA1402-00.

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Figure 1. Schematic illustration of the optoelectronic device prepared with the oxidized graphene–organic dye hybrid structure. The right panel shows the epoxide, hydroxyl, and carboxyl groups formed on graphene after UV/ozone exposure. The upper panel shows the chemical structure of the Rhodamine 6G (R6G) organic dye used in this study.

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Figure 2. (a) High-resolution XPS C1s spectra of the pristine graphene and graphene oxidized by UV/ozone exposure for 15 or 30 s. (b) The atomic ratio of the oxygen-containing groups (epoxide, hydroxyl, and carboxyl groups). (c) Transfer characteristics of the transistors based on the pristine graphene and UV/ozone-oxidized graphene channels. (d) Change in the Dirac voltage and hysteresis of the transistors after UV/ozone exposure onto the graphene channel. (e) Raman spectra of the R6G film deposited onto pristine graphene and graphene oxidized by UV/ozone exposure for 15 or 30 s.

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Figure 3. (a–c) Illumination-dependent transfer characteristics of the graphene–organic dye hybrid transistors based on the pristine graphene and the graphene oxidized for 15 or 30 s under different illumination powers at a fixed incident illumination wavelength of 520 nm (measured under ambient conditions). (d) Dirac voltage shift, (e) photocurrents at the Dirac voltages under dark conditions, and (f) R as a function of the illumination power.

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Figure 4. (a–b) Illumination-dependent transfer characteristics of the graphene–organic dye hybrid transistors based on (a) the pristine graphene and (b) graphene oxidized for 30 s measured under ambient or vacuum conditions. The illumination wavelength was 520 nm. (c) Dirac voltage shifts in the hybrid transistors based on the pristine or UV/ozone-oxidized graphene measured under ambient or vacuum conditions\. (d) Schematic band structure explaining the environment-dependent photoresponse characteristics measured from the hybrid structures.

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Figure 5. (a) Schematic illustration and circuit diagram of the logic inverter with light-controllable signal inversion. (b–c) Voltage transfer characteristics of the logic inverters upon illumination onto (b) the graphene–organic dye channel or (c) the encapsulated graphene–organic dye channel.

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TOC Figure

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Photoresponse of Physically Oxidized Graphene Sensitized by an

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