Enhanced Luminescence and Photocurrent of

Electron. Mater. Lett., Vol. 0, No. 0 (0000), pp. 1-8 DOI: 10.1007/s13391-015-4496-0

Enhanced Luminescence and Photocurrent of Organic Microrod/ZnO Nanoparticle Hybrid System: Nanoscale Optical and Electrical Characteristics Guru P. Neupane,1,2 Krishna P. Dhakal,1,2 EunHei Cho,3 Bong-Gi Kim,4,5 Seongchu Lim,1,2 Jubok Lee,1,2 Changwon Seo,1,2 Young Bum Kim,1,2 Min Su Kim,1 Jinsang Kim,4 Jinsoo Joo,3,* and Jeongyong Kim1,2,* 1

Center for Integrated Nanostructure Physics, Institute for Basic Science (IBS), Sungkyunkwan University, Suwon 440-746, Korea 2 Department of Energy Science, Sungkyunkwan University, Suwon 440-746, Korea 3 Department of Physics, Korea University, Seoul 136-713, Korea 4 Department of Materials Science and Engineering, Macromolecular Science and Engineering and Chemical Engineering, University of Michigan, MI 48109, USA 5 Department of Organic and Nano System, Konkuk University, Seoul 143-701, Korea (received date: 14 November 2014 / accepted date: 3 March 2015 / published date:

)

We studied the enhanced photoluminescence (PL) and photocurrent (PC) of 1,4-bis(3,5-bis(trifluoromethyl)styryl)-2,5-dibromobenzene (TSDB) microrods decorated with ZnO nanoparticles (NPs). Chemically synthesized crystalline ZnO NPs with an average size of 40 nm were functionalized with (3aminopropyl)trimethoxysilane to result in the chemical bonding of the NPs onto the surface of the TSDB microrods. We observed a 2-fold PL enhancement in the ZnO/TSDB hybrid microrods compared with the PL of the pure TSDB microrods. In addition, PC measurement carried out on the TSDB and ZnO/TSDB hybrid microrods at two different excitation wavelengths of 355 nm and 405 nm showed the significant enhancement of the PC from the hybrid system, where the resonant excitation of the laser (355 nm) corresponding to the absorption of both ZnO and TSDB caused ~3 times enhancement of the PC from the ZnO/TSDB hybrid microrods. Keywords: TSDB, ZnO, photoluminescence, energy transfer, photocurrent

1. INTRODUCTION Inorganic/organic hybrid structures have been widely studied to investigate their optical,[1,2] electrical,[3,4] mechanical,[5,6] thermoelectric,[7,8] and magnetic[9,10] properties for device applications. 1,4-bis(3,5-bis(trifluoromethyl)styryl)2,5-dibromobenzene (TSDB) are π-conjugated organic molecules incorporating halogen atoms, fluorine (F), and bromine (Br) in the backbone. The incorporation of F and Br is uncommon in most other π-conjugated semiconducting materials,[11,12] and their presence in the TSDB molecule is considered to induce a close-packed molecular structure and enhance the electronic density as a result of the hydrophobic *Corresponding author: [email protected] *Corresponding author: [email protected] ©KIM and Springer

and heavy atom effect, respectively.[13] As a π-conjugated organic crystal incorporated with heavy halogen atoms, TSDB is a potential material for optoelectronic applications. Recently, the polychromatic optical waveguiding of TSDB nanowires was demonstrated.[13] In addition, TSDB has been introduced as an interesting organic material which emits a blue-green light and exhibits an efficient waveguiding nature to its photoluminescence (PL) and Raman signal in its microplate structure.[14] However, its electrical properties are influenced by the close packed molecular structure in addition to the halogen atoms in the backbone; this effect could be interesting and it has not been investigated. The hybridization of organic materials is also of interest for the tuning of optoelectronic properties and the use of inorganic nanoparticles for the hybridization of organic materials is one of the fascinating techniques used to study hybrid nanomaterials.[1,2] Among the many kinds of

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inorganic materials, ZnO is a nontoxic, wide-band gap semiconductor that has been used for various applications such as light emitting diodes (LEDs),[15,16] solar cells,[17,18] field effect transistors,[19,20] and medicine.[21,22] Nowadays, it is used as an alternative to TiO2 as a photocatalytic material because of the wide range of geometrical structures within its nanomaterial.[23,24] The effectiveness of organic solar cells and light emitting diodes (LED) using ZnO nanorods or nanowires hybridized with organic polymers has been extensively studied previously.[25-28] However, no previous study has used crystalline ZnO NPs to investigate the πconjugated organic/inorganic hybrid structure. In general, many inorganic/organic structures still lack energy transfer based phenomena. Usually, energy transfer phenomena are used for bio chemical sensing[29-31] and LEDs.[32,33] The photocurrent (PC) mechanism in conducting organic materials often differs drastically from their crystalline inorganic counterparts, and many aspects are still unclear. For future optoelectronic applications, it is important to understand the optical and electrical phenomena owed to the interface between the inorganic and organic molecular materials. In this work, we explored the optical and electrical properties of the pure TSDB microrods and TSDB microrods decorated with ZnO NPs. The ZnO/TSDB hybrid microrods exhibited a two-fold PL enhancement compared with the pure TSDB microrods. We also measured the PC of the pure TSDB and ZnO/TSDB hybrid microrods, and the ZnO/TSDB hybrid microrods demonstrated substantially enhanced PC values in comparison to the pure TSDB microrods.

2. EXPERIMENTAL PROCEDURE All the chemicals were purchased from Sigma Aldrich and Fisher Scientific. Subsequently, the TSDB microrod was prepared using the chemicals 1,1'-[(2,5-dibromo-1,4-phenylene) bis(methylene)] bis [1,1,1-triphenyl phosphoniumdibromide and 3,5-bis(trifluoromethyl) benzaldehyde according to procedures described previously.[13] Briefly, a solution of two compounds 1,1'-[(2,5-dibromo-1,4-phenylene) bis(methylene)] bis [1,1,1-triphenyl phosphonium dibromide (1 g, 1.057 mmol) and 3,5-bis(trifluoromethyl) benzaldehyde (0.51 g, 2.114 mmol) was prepared in anhydrous tetrahydrofuran. Potassium tert-butoxide (0.3 g, 2.5 eq) was proportionally added to this solution mixture over a period of 10 min. Subsequently, the mixture was warmed to room temperature and maintained with overnight stirring. This mixture was subsequently poured into water and the precipitate was collected and washed with methanol. Subsequently, using flash column chromatography with a chloroform eluent, the TSDB powder was recrystallized. Finally, the TSDB powder dissolved in chloroform was filtered using a syringe filter. The TSDB molecules in the supersaturated solution self-assembled into microrods within a few minutes.

We prepared the ZnO NPs by employing a chemical method that used two different solutions of (a) zinc sulfateheptahydrate (0.3 M) and 0.2% (v/V) Triton X-100 (C14H22O(C2H4O)n) and (b) ammonium bicarbonate (0.75 M) and 0.2% (v/V) Triton X-100. The solutions were prepared in distilled water (DI). The reaction solution was prepared by the drop-wise mixing of the zinc sulfateheptahydrate solution with the solution of ammonium bicarbonate. This solution mixture was vigorously stirred for 5 h. Subsequently, the solution was left undisturbed overnight for precipitation to occur. A clear white precipitate formed and this was filtered and washed with DI water several times to remove the sulfate (SO42−) residues. The resultant precipitate was vacuum dried at 90°C for 2 h and small crystalline basic zinc carbonate (ZnCO3·2Zn(OH)2·H2O) particles were obtained. Finally, the crystalline basic zinc carbonate was calcined for 2 h at 350°C to obtain the ZnO NPs. The ZnO NPs were subsequently functionalized with 0.2% (v/V) (3-aminopropyl) trimethoxysilane (APTMS) through vigorous stirring in a chloroform solution for 4 h. Subsequently, the TSDB microrods were decorated with APTMS-functionalized ZnO NPs in order to obtain an inorganic/organic hybrid structure. This was performed by slowly stirring the solution mixture for 10 min and incubating the solution for 12 h. The schematics of the preparation and functionalization of the ZnO NPs in addition to the decoration on the TSDB microrods are provided in the supplementary material (Fig. S1). For the micro Fourier transform infrared spectroscopy (micro FTIR), laser confocal microscopy (LCM) PL experiments, and Raman spectroscopy measurements, a residue-free glass slide with a thickness of 1 mm (DURAN GROUP, Germany) and a cover glass with a thickness of 500 μm (MENZEL-GLASER, Germany) were used. To remove the inorganic/organic residues of the glass substrate, the glass slide and cover glass were initially cleaned in KOH and acetone solution and this was followed by the piranha cleaning process.[34] To measure the PC, we used a gold patterned structure, which was fabricated using an e-beam lithography technique. Chrome gold was deposited onto a 300 nm thick SiO2 layered silicon substrate to produce electrodes. Subsequently, the pure TSDB and ZnO/TSDB hybrid microrods were directly transferred onto the patterned substrate by drop casting the respective solutions. The ultraviolet/visible (UV/Vis.) absorption spectra of the ZnO NPs, TSDB microrods, and ZnO/TSDB hybrid microrods were measured in a chloroform solution, using a V-670 spectrometer at room temperature. Field emission scanning electron microscopy (FESEM) images were captured using 5 - 20 kV field emission voltages (JEM-2100F, JEOL Corp.). The FTIR measurements of the individual TSDB and ZnO/ TSDB hybrid microrods were performed using the micro FTIR system, Hyperion-2000. In order to analyze the IR spectra of the TSDB microrod decorated with the APTMS-

Electron. Mater. Lett. Vol. 0, No. 0 (0000)


functionalized ZnO NPs, micro FTIR measurements of the pure APTMS and APTMS-functionalized ZnO NPs were also performed using the same micro FTIR system. The PL spectral mapping of the pure TSDB and ZnO/TSDB hybrid microrods was performed by excitation with a diode laser, λex = 355 nm. The 514 nm laser line of the He-Ne gas laser was used for the Raman excitation on both the pure TSDB and ZnO/TSDB hybrid microrods. An oil-immersion objective lens with a 1.3 numerical aperture (NA) was used in the inverted geometry to excite the sample. The same objective lens was used to collect the PL and Raman signals. This signal was guided to a 50 cm long monochromator (equipped with a cooled CCD) through a multimode optical fiber with a 100 μm core diameter, which functioned as a confocal detection pinhole. For the measurement of the PC, excitation lasers (λex=355 and 405 nm (both diode lasers)) were focused at various positions on the single microrod using an air objective lens of 0.6 NA, and the generated PC

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was recorded using a source meter (Keithley 2450).

3. RESULTS AND DISCUSSION 3.1 Preparation of TSDB microrods decorated with ZnO NPs Figure 1(a) shows a schematic of the molecular structure of the π-conjugated organic molecule TSDB, APTMS, and APTMS-functionalized ZnO NPs in addition to the chemical bonding between the functionalized ZnO NPs and the TSDB molecules. In the APTMS-functionalized ZnO NPs, Zn-OSi covalent bonds were formed at the surface of ZnO NPs, leaving the amine groups at the periphery to form a shell-like structure.[35] The amine shell surrounding the APTMSfunctionalized ZnO NPs has a crucial function for the decoration of the ZnO NPs onto the TSDB microrods; a halogen bond is formed in-between the nitrogen of the amine and the halides of the TSDB molecules, as represented by

Fig. 1. (a) Schematic diagram showing the molecular structure of the APTMS, TSDB, and APTMS-functionalized ZnO NPs and the mechanism of the chemical bonding during the fabrication of the ZnO/TSDB hybrid microrods. (b), (c) and (d) represent the micro FTIR absorption spectra of the TSDB microrod (black curve), ZnO/TSDB hybrid microrod (red curve), and ZnO NPs (blue curve). The modification of the C-F vibration in the TSDB as a result of the attachment of the APTMS-functionalized ZnO NPs is shown separately in the inset. (e) FESEM image of ZnO/TSDB hybrid microrod. (f) Magnified FESEM image of ZnO/TSDB hybrid microrod in the marked region of Fig. 1(e).


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the dotted line in the schematic in Fig. 1(a). We performed micro FTIR measurements for the ZnO NPs, TSDB microrod, and ZnO/TSDB hybrid microrods on the solid sample and studied the different vibrational bands, as shown in Figs. 1(b-d). As demonstrated in Figs. 1(b-d), the IR spectra of the TSDB and ZnO/TSDB hybrid microrods are normalized at the peak at 1280 cm−1 in which we observed the strong IR band intensity of the ZnO/TSDB microrod (red curve) at 760 cm−1. However, the IR bands of the TSDB microrod (black curve) and ZnO NPs (blue curve) exhibit negligible IR intensity at that peak position. This relative enhancement of the IR band of the ZnO/TSDB hybrid microrod at 760 cm−1 can be attributed to the N-Br stretching vibration.[36] This confirms the formation of a chemical bond between the APTMS-functionalized ZnO NPs and the TSDB microrod. In addition, we observed the modification of the C-F vibration in the pure TSDB microrod when the APTMS-functionalized ZnO NPs were hybridized with the TSDB microrod, as shown in the inset of Fig. 1(b). This effect could originate from the formation of the N-F bond in the TSDB microrods decorated with ZnO NPs; however, the presence of the N-F bond is not clear because of the overlap of its peak positions with the C-H vibration.[37] We also found evidence for the formation of the N-F bond in the ZnO/TSDB hybrid microrod; the red shifting of the Raman peak positions correlate with the -CF3 stretching vibration of the pure TSDB microrod. The details of the Raman spectra are shown in the supplementary material (Fig. S2 and Table S1). The IR peaks at 813 cm−1, 1000 cm−1, 1436 cm−1, 1563 cm−1, 2880 cm−1, 2938 cm−1, 3292 cm−1, 3367 cm−1, and 3438 cm−1 for the ZnO/TSDB hybrid microrod are represented by the stars in Figs. 1(b-d). These peaks originate from the APTMSfunctionalized ZnO NPs, which also suggests the formation of the TSDB microrod decorated with ZnO NPs. The details of the IR spectra in relation to the APTMS-functionalized ZnO NPs and the respective peak positions of the TSDB and ZnO/TSDB hybrid microrods are summarized in the supplementary material (Fig. S3 and Table S2). Figs. 1(e,f) show the FESEM image of the micrometer-sized TSDB microrod decorated with ZnO NPs with a size of ~40 nm. 3.2 Photoluminescence and photocurrent study Figure 2 displays the PL spectral mapping images of the TSDB and ZnO/TSDB hybrid microrods that were obtained using the same experimental conditions as above. We excited the TSDB and ZnO/TSDB hybrid microrods with a 355 nm laser to simultaneously excite both the ZnO and TSDB in the ZnO/TSDB hybrid system, and we compared the measured values of the PL emission with the comparable size of the TSDB and ZnO/TSDB hybrid microrods. The PL spectral images and corresponding PL spectra are displayed in Figs. 2(a-c). An approximate two-fold PL enhancement can be observed for the ZnO/TSDB hybrid microrods

Fig. 2. PL spectral mapping images of the (a) pure TSDB microrod and (b) ZnO/TSDB hybrid microrod with an excitation wavelength 355 nm. (c) The comparison of the average PL spectra for the pure TSDB microrod (black curve) and ZnO/TSDB hybrid microrod (red curve), demonstrating a ~2-fold enhancement of the PL signal from the hybrid system. The overlap of the PL of the ZnO NPs with the absorption of the TSDB is shown in the inset of Fig. 2(c).

compared with the pure TSDB microrods. This effect is attributed to the energy transfer from the ZnO NPs to the TSDB molecules. As shown in the inset of Fig. 2(c), the spectral overlap of the optical absorption of the TDSB microrods with the PL spectra of the ZnO NPs suggests the onset of the energy transfer in the ZnO/TSDB hybrid microrods, which could have achieved the PL enhancement. We note that the PL of the ZnO NPs on the surface of the TSDB microrods was completely supressed. This also suggests that the energy transfer effect resulted in the enhancement of the PL in the hybrid ZnO/TSDB hybrid microrods. We conducted time-resolved photoluminescence (TRPL) measurements to compare the exciton lifetimes of the TSDB and ZnO/TSDB microrods, and the result (Fig. S4) indicates that a slightly slower exciton decay occurs in the ZnO/TSDB hybrid microrod and provides additional support to the energy transfer. We note that the PL enhancement and the longer exciton decay of the ZnO/ TSDB hybrid microrod may be due to other reasons such as the passivation of the TSDB microrod surface by the ZnO NPs. While the exact origin is not clear, we believe that the observed PL enhancement of the ZnO/TSDB hybrid microrod suggests that the improvement of the optical properties is achieved by hybridization. Figure 3(a) shows the I-V characteristics of the pure TSDB microrod. The I-V curve of the pure TSDB microrod exhibits



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Fig. 3. (a) I-V curve of the pure TSDB microrod. (b) The schematic band diagram for the pure TSDB and Au contact. (c) I-V curve of the ZnO/ TSDB hybrid microrod. (d) Schematic band diagram for the ZnO/TSDB hybrid structure.

Ohmic behavior, with the exception of a small current at zero bias voltage. Based on the linear characteristics of the I-V curve, the Fermi levels of the TSDB and Au electrode are believed to be at similar levels, as described by the schematic in Fig. 3(b). This suggests that TSDB is primarily a hole-transporting p-type material, similar to other organic crystals.[38] The recorded current at zero bias could be a thermally excited current, similar to previously reported results of organic semiconductors.[39,40] The I-V curve of the hybrid ZnO/TDSB microrod is shown in Fig. 3(c). The curve demonstrates the characteristics of the Schottky diode and it can be observed that the zero bias current in the ZnO/ TSDB hybrid microrod has disappeared. The observed sudden change in the I-V characteristics, from Ohmic to Schottky, by hybridization in the ZnO/TSDB microrod, can be attributed to the space charge limited current (SCLC) effect as a result of the introduction of the ZnO NP between the TSDB and Au electrode. A schematic of the band diagram for the ZnO/TSDB hybrid structure and the Au electrode is shown in Fig. 3(d). The occurrence of the SCLC current requires at least one contact and the current has superior carrier injecting properties to provide an inexhaustible carrier source. The characteristic non-linear behaviour of the

I-V curve in our results suggests that hole injection from TSDB to the ZnO has occurred in the hybrid structure of the n-type ZnO NP and p-type TSDB microrod. This process resulted in the band bending at the interface of the ZnO and TSDB and it may have resulted in the SCLC effect in the large work function metal electrode. Previously, a similar SCLC effect was demonstrated using the film of the p-type organic material, anthracene, and the n-Si interface.[41] The onset of the current at ~1.1 V also correlates with the offset energy of 0.9 eV between the Au Fermi level and the conduction band edge of ZnO. As additional support, the I-V curve displayed a I ~ V5 trend (Fig. S5) which is consistent with that SCLC displays; the dependence of the current on some power of the bias voltage, mostly 2 or higher.[41,42] Figures 4(a,b) display the epi-fluorescence images of the PC devices of the pure TSDB and ZnO/TSDB hybrid microrods which were obtained by excitation with a 355 nm laser. For the measurement of the PC, I-V curves were obtained both with and without a laser light (with a typical power of 300 μW) fixed onto the sample at five different positions, as indicated in Figs. 4(a,b). Two different wavelengths of 355 nm and 405 nm were used for the PC measurement. The comparison of the PC measurement


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Fig. 4. Comparison of the PC measurement of the pure TSDB and ZnO/TSDB hybrid microrods obtained using excitation lasers with wavelengths of 355 nm and 405 nm. Epi-fluorescence image of (a) pure TSDB microrod and (b) ZnO/TSDB hybrid microrod excited by the 355 nm laser. The insets show the optical images of the corresponding PC devices. The arrows in the fluorescence images represent the positions of the laser light that was focused on the microrods for the PC measurement. I-V curves for the (c) Pure TSDB microrod and (d) ZnO/TSDB hybrid microrod with and without laser excitation of 355 nm obtained from five different positions. (e,f) PC measurement with 405 nm excitation obtained from five different positions.

results for the pureTSDB and ZnO/TSDB hybrid microrods with 355 nm laser excitation is shown in Figs. 4(c,d). We compared the PC of the pure TSDB and ZnO/TSDB hybrid microrods at 5 V bias voltage and we observed that the PC of the ZnO/TSDB hybrid structure demonstrated approximately a three-fold enhancement. This enhancement of the PC is attributed to the amplified absorption of the 355 nm laser

light by the ZnO NPs decoration on the TSDB microrod. Since the ZnO and TSDB both demonstrate significant absorption of the 355 nm laser (as noted in Fig. S3(a)), this will increase the net PC in the ZnO/TSDB hybrid system. In addition, the I-V curves obtained from the five different positions on the pure TSDB microrod (as shown in Fig. 4(c)) do not demonstrate the PC variation, while the ZnO/TSDB



hybrid microrod exhibits locally varying PC when excited with a 355 nm laser, as shown in Fig. 4(d). This spatial variation of the PC measurement of the ZnO/TSDB hybrid microrod could be attributed to the inhomogeneous distribution of the ZnO NPs on the TSDB microrod. This could have caused the variation in the absorption efficiency of the 355 nm excitation laser light. We also excited the same pure TSDB and ZnO/TSDB hybrid microrod with a 405 nm laser and obtained PC measurements, as shown in Figs. 4(e,f). In this case, we observed the PC enhancement of the ZnO/TSDB hybrid system. An approximate two-fold enhancement was achieved at 5 V bias voltage. For this, the ZnO NPs on the TSDB surface may have resulted in the larger absorption because of enhanced scattering. With 405 nm laser excitation, we did not observe the local variation of the PC in the ZnO/TSDB hybrid microrod. This could be the result of the excitation of the TSDB molecules in the ZnO/TSDB hybrid structure with the 405 nm laser, as suggested by the absorption spectrum shown in Fig. S3(a). We determined that a 355 nm excitation wavelength achieves a superior PC enhancement for the ZnO/TSDB hybrid microrod when compared with the 405 nm laser excitation. This is due to the common optical absorption of the excitation wavelength of 355 nm by the ZnO NPs and TSDB microrods, while the 405 nm laser light is only absorbed by the TSDB molecules.

4. CONCLUSIONS We prepared ZnO/TSDB hybrid microrods and this was confirmed using micro FTIR, confocal Raman, and FESEM analysis. The PL and PC properties of the ZnO/TSDB hybrid system were compared with those of the pure TSDB microrod. The PL measurement demonstrated a two-fold enhancement for the ZnO/TSDB hybrid microrod. This was possibly due to the energy transfer phenomenon and the surface passivation of the TSDB microrod by the ZnO NPs. The PC measurements were performed with two different excitation wavelengths of 355 nm and 405 nm. A higher PC was exhibited by the ZnO/ TSDB hybrid microrod, where the 355 nm laser excitation demonstrated improved PC enhancement because of the common absorption by the ZnO NPs and TSDB molecules for light with this wavelength.

ACKNOWLEDGEMENTS This work was supported by IBS-R011-D1 and National Research Foundation grants (No. 2012R1A1A2A10043220). Electronic Supplemental Materials: The online version of this article (DOI: 10.1007/s13391-015-4496-0) contains supplemental materials.

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