Pyramid-Shape Tris(8-hydroxyquinoline) Aluminum Schottky Diode

Article pubs.acs.org/JPCC

Pyramid-Shape Tris(8-hydroxyquinoline) Aluminum Schottky Diode Shih-Shou Lo* and Shu Hao Sie Department of Photonics, Feng-Chia University, 100, Wenhwa Road, Seatwen, Taichung, 40724, Taiwan, Republic of China ABSTRACT: We fabricated pyramid-shape tris(8-hydroxyquinoline) aluminum (Alq3) via a microemulsion process. The Alq3 Schottky diode with a single pyramid-shape Alq3 across the Ag electrodes was demonstrated. Rectifying and negative differential resistance (NDR) behavior with large peak-to-valley (PV) ratio of the device was obtained. The NDR behavior of the device was presented when the forward bias exceeded 4 V, and the PV ratio in the current was more than 10 at room temperature. The NDR behavior in the device was discussed. This work further suggests that the organic semiconductor can be synthesized to further explore applications for advanced electronic device.

1. INTRODUCTION Organic semiconductor materials attract attention because of their huge potential as an active layer in the optical and electronic applications, such as organic light-emitting diodes (OLED) and organic solar cells.1,2 Among the existing organic semiconductor materials, tris(8-hydroxyquinoline) aluminum (Alq3) is a crucial materials for organic optoelectronic devices because the Alq3 was used as electronic transport and emitting material in the OLED in 1987.3 Several studies in this field have focused on the optimization of device performance for efficiency and long-term stability or on the understanding of charge-transport properties.4,5 It is well known that the small size of nanostructured materials may result in various optical, electronic, magnetic, and mechanical properties: therefore, there are more suitable for noble application. However, considerable effort in the development of nanomaterials has been devoted to design and researches of inorganic materials. Studies related to organic or organometallic nanomaterials are limited.6,7 With the rapid development of nanoscience and nanotechnology, substantial effort is dedicated to the controlled synthesis of Alq3 nanostructures, and to find the potential applications of Alq3 nanostructure is a challenge. In this study, we synthesized pyramid-shape Alq3 using a microemulsion process and fabricated a Schottky diode with a single pyramid-shape Alq3 across paired Ag electrodes. The rectifying characteristic and negative differential resistance (NDR) behavior of the proposed device is obtained. We describe the results with associated discussion to find the possible mechanism.

appropriate amount of sodium dodecyl sulfate (SDS) dissolved in H2O (16 mL). The SDS compositions for pyramid-shape Alq3 are thereby as follows: recipe A: 10 mg SDS; recipe B: 30 mg SDS; and recipe C: 50 mg SDS. The resulting two-phase mixture was transformed into an emulsion by ultrasonication and vigorous stirring. The emulsion was heated in a water bath at 55−60 °C for ∼3 h to evaporate CHCl3. Finally, the pyramid-shape Alq3 was collected by a centrifugation mechanism and redispersed in deionized water. The single pyramid-shape Alq3 was placed across the prefabricated Ag electrodes using the droplet technique. Measurements of photoluminescence (PL) were performed under an ambient atmosphere at room temperature in a He:Cd laser (325 nm) and a monochromator. Measurement of IV characteristics was performed at room temperature in Kiethely 236 and a probe platform measurement system with high accuracy.

3. RESULTS AND DISCUSSION 3.1. Morphology and Structural Properties of Pyramid-Shape Alq3. Typical SEM images of Alq3 morphology variation using this microemulsion technique are shown in Figure 1. For comparison, Figure 1a also presents the SEM image of original Alq3 powder with irregular shape. Following the recipe A process, the irregular shapes Alq3 were bound together. This resulted in a rod shape with a rough surface, as shown in Figure 1b. Following the recipe B process, a pyramid-shape Alq3 with small cavities on the surface was formed, as shown in the Figure 1c. Following the recipe C process, a typical pyramid-shape Alq3 with smooth surface was fabricated, as shown in Figure 1d. Clearly, the concentration of SDS influences the morphology of Alq3 when the Alq3 was processed with microemulsion process.

2. EXPERIMENTAL METHOD Alq3 powders and pyramid-shape Alq3 were synthesized in our laboratory following a slightly modified synthetic route derived from relevant literature.8−10 Normal microemulsion was used to fabricate pyramid-shape Alq3. The Alq3 (20 mg) powder was dissolved in 1 mL of CHCl3, followed by the addition of an © 2012 American Chemical Society

Received: November 21, 2011 Revised: July 3, 2012 Published: July 9, 2012 16122

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Figure 3. Schematic illustrations of pyramid-shape Alq3 growth mechanism.

Figure 1. SEM image of Alq3 morphology synthesized via different process. (a) As-synthesized Alq3 powder, (b) recipe A, (c) recipe B, and (d) recipe C.

Figure 4. Comparison of Alq3 FTIR spectrum for different morphology.

Figure 5. Comparison of photoluminescence spectra of Alq3 synthesized via various recipes.

Alq3 powder and SDS powder. These differences were clearly observed for small angles below 10° and in the region of 20 and 25°. It is possible to compare these crystal data with the results of other researchers. Brinkmann et al. reported three different crystalline structures called α, β, and γ phases.11 Powder X-ray diffraction patterns of Alq3 powder exhibited reflections at 2θ οf 6.4°, which was indexed to the [001] reflection peak of the meridional α phase. The published data for the α-phase are identical to those of Alq3 powders. The XRD spectra also

Figure 2. Comparison of Alq3 XRD pattern for various morphologies.

The crystallographic data of the samples were determined using X-ray powder diffraction to investigate the structural properties of morphology differences, as shown in Figure 2. For comparison, Figure 2 also presents the XRD spectra of original 16123

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Figure 6. (a) Photographic image of pyramid-shape Alq3 across the Ag pads for IV measurement under visible light excitation. (b) Photographic image of pyramid-shape Alq3 across the Ag pads for IV measurement under UV light excitation. Figure 9. Modified energy band diagram of the pyramid-shape Alq3 diode under interface tunnel.

concentration of Alq3 in the micelles to increase and eventually supersaturate, which leads to nucleation. Once the chloroform in a micelle was depleted, the nuclei moved from the inner core of the micelle to the anionic headgroups of SDS (SO4−). They grew into pyramid-shape structures because the growth rate of [001] direction is faster than that of other directions, which is mainly driven by crystal packing forces and π−π stacking interactions among adjacent Alq3 molecules.12 Because SDS provides molecules excessive attracting force of molecules during the pyramid-shape formation, the chemical bonding structures of the fabricated pyramid-shape Alq3 were investigated. Figure 4 shows the FTIR spectra of the original and pyramid-shape Alq3. For comparison, Figure 4 also shows the FTIR spectra of SDS powder. The Al−N stretching was found at 418 cm−1, and Al−O stretching was found at 458, 522, and 542 cm−1. The C−H bending was found at 577, 648, 748, 787, 804, 825, and 868 cm−1. No substantial change in chemical bonding structures of all samples was observed within the detection limit of FTIR. This indicates that no structural change occurred in Alq3 chemical bonding. 3.3. Photoluminescence Spectra of Pyramid-Shape Alq3. To investigate the fact that the morphology variation influences the room-temperature PL (RT-PL) of Alq3, Figure 5 shows the RT-PL of Alq3 with various morphologies. A maximal intensity was located at 504 nm of original Alq3 powder, which was that of the α-phase Alq3 reported by Colle at al.16 A spectral red shift (35 nm) was observed in the pyramid-shape Alq3 when the SDS concentration is enough to complete the morphology transformation. Two explanations

Figure 7. Current I as a function of applied bias Vb for Alq3 Schottky diode under dark condition.

revealed that the SDS concentration influences the crystalline structure of Alq3. The diffraction peaks of the pyramid-shape Alq3 exhibited almost the same 2θ location as those of SDS. It also reveals that the headgroup of SDS was attached on the pyramid-shape Alq3. Figure 2b,c displays magnified spectra of the marked areas in Figure 2a, which clearly show the α-Alq3 diffraction peaks. 3.2. Proposed Growth Mechanism of Pyramid-Shape Alq3. To understand further the formation of pyramid-shape Alq3, a proposed growth mechanism of pyramid-shape Alq3 is plotted in Figure 3. First, a normal microemulsion of CHCl3 in H2O was formed by vigorous ultrasonication. When the emulsion was heated to the temperature of 60 °C in a water bath, the chloroform in some micelles slowly evaporated into the atmosphere because of its low boiling point, causing the

Figure 8. Energy band diagram of the fabricated diode (a) before contact and (b) after contact. 16124

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resistance was discussed to understand further the origin of the negative resistance in the obtained IV characteristics. The negative resistance is usually observed in the tunnel diodes, the electron/hole resonant tunneling diodes, and molecular rectifier.17 The tunnel diode was attributed to the interband tunneling between the conduction (LUMO) and valence (highest occupied molecular orbital, HOMO) band. The energy band diagrams of the diode before and after contact under zero bias are shown in Figure 8a,b. Because the HOMO of Alq3 and the work function of Ag are not overlapped, the interband tunnel in the tunnel diodes cannot appear in our fabricated device. Under the forward bias, the electrons will flow through two barriers including a 1.7 eV Schottky barrier for the electron injection from Ag electrodes to the Alq3 LUMO and a band bending barrier for the electron injection from the Alq3 LUMO to the Ag electrode because of the energy offset between them. However, the distance of the two barriers was ∼40 μm, which is much larger than the barrier distance at which the electron resonant tunneling may occur (approximately several nanometers). For the hole transport under the positive bias, a 0.6 eV Schottky barrier was observed for the holes injection from the Ag electrode to the HOMO of Alq3. In general, the electrons/holes resonant tunneling occurs in two or more barriers and one well, with at least one bound energy state structure. Single 0.6 eV Schottky barriers cannot support the resonant tunneling effect. In 1974, Aviram and Ratner introduced the concept of a molecular rectifier.18 The NDR behavior can be observed in some molecular rectifier.19 There always exist two types of molecular rectifier: normal directional rectification and reverse rectification. It is worth noting that Pan et al. (2011) design various Aviram−Ratner rectifiers based on the donor-σ bridgeacceptor (D-σ-A) molecules to examine the rectifying performances by the first-principles method.20 They reported the reverse rectification behavior in the D-σ-A molecules. When the end groups of D-σ-A were replaced with function groups to increase the spacer length between metal electrodes and electroactive moieties. The rectifying performance shows the most obvious enhancement when increasing symmetrically the length of the end group. Yee et al observed the inverse rectification in a metal−molecule (donor−acceptor molecular)−metal heterojunctions.21 They used single-level coherent tunneling model to explain the differential conductance and rectification. On the basis of their results, the various side groups attached to molecules may be use to modified its transport behavior in a controlled manner and improve/add a particular functionality. Recall the synthesis of pyramid-shape Alq3 requires using a microemulsion process. Therefore, the pyramid-shape Alq3 may be covered by a thin SDS headgroup layer. It is well known that the Alq3 is a typical electron-transporting organic semiconductor. In the device, the holes injection barrier is lower than the electronics injection barrier. The mobility of hole is two orders in magnitude lower than the electron mobility. Therefore, the injected holes become accumulated resulting in the formation of the space charge and consequently higher electrical field in the Alq3 layer. This means that the holes accumulation and its injection may play an important role in the observed NDR properties. When the forward bias increases, the current of the device increases. The LUMO level of the Alq3 will approach the neighboring empty LUMO level. If the LUMO level becomes higher than the unoccupied

are offered for red-shift emission in Alq3. Chen et al. reported that the electron-withdrawing substituent causes a blue shift, whereas electron-donating groups cause a red shift in the emission properties in Alq3.14 Higginson et al. reported redshift emission of Alq3 powder caused by the formation of aggregates or crystals which promote formation of ground-state complexes.15 A progressive red shift with increasing crystallographic disorder was demonstrated by Colle.16 In this study, the red-shift emission of pyramid-shape Alq3 may be attributed to the changes in the crystal structural with the anionic headgroup of SDS. 3.4. Schottky Diode with a Single Pyramid-Shape Alq3 Across Ag Electrodes. To demonstrate further the application of the pyramid-shape Alq3, a metal/organic semiconductor/metal diode was fabricated using a single pyramid-shape Alq3 across paired Ag electrodes on the B270K glass. Figure 6a,b displays the photo image of the single pyramid-shape Alq3 for IV measurement under different light excitations. The current will flow through the probe, the Ag electrode, the pyramid-shape Alq3, the Ag electrode, and the probe under a forward bias. For the Ag/Alq3 interface, it is generally by using Richardson−Schottky (RS) thermionic emission or Fowler−Nordheim tunneling model. For V ≫ 3 kT/q, the current density by the RS thermionic emission is subsequently written as17 ⎛ qV − IR ⎞ I = I0 exp⎜ ⎟ ⎝ ηkT ⎠

(1)

where η is the ideality factor, R is the series resistance, and K is Boltzmann’s constant. The saturation current I0 is derived by ⎛ φ ⎞ I0 = A*ST 2 exp⎜ − B ⎟ ⎝ kT ⎠

(2)

and φB = Wm − E LUMO − Δ

(3)

where A* is the effective Richardson constant, Wm is the work function of metal, ELUMO is the lowest unoccupied molecular orbital (LUMO) of Alq3, Δ is the dipole barrier formation, and S is the Schottky contact area. The most noteworthy feature is that the IV characteristic of the devices formed by this process displayed a rectifying behavior, as shown as in Figure 7. The diode exhibited very low (1 × 10−11 A, equivalent to 4 × 10−5 A/cm2 at −10 V) reverse currents, and the breakdown voltage was −15 V. In forward bias, the IV characteristic curve exhibited a turn-on voltage of 0.5 V. A current of 0.02 nA at 0.5 V forward bias was received. The diode can bear an applied voltage of up to 4 V. When IV characteristics of the Ag/Alq3 single Schottky diode were measured under dark condition, the current exhibited a strong and almost exponential increase with forward bias. The nonlinearity of I−V curve was caused by the Schottky barrier that formed between the Alq3 and Ag. The Ag work function was 4.3 eV, and LUMO of Alq3 was 3 eV. The ideality factor of the device exceeds 2 from the slope of ln(I) versus V plot. The higher ideality factor may be caused by the existence of interfacial layer and surface states. This also indicates that the performance of the device must be improved. 3.5. Possible Mechanism for Negative Differential Resistance in Pyramid-Shape Alq3 Diode. A negative resistance behavior with a peak-to-valley (PTV) ratio in current of more than 10 was clearly observed in the IV curve when the forward bias exceeded 4 V. The mechanism of the negative 16125

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(21) Yee, S. K.; Sun, J.; Darancet, P.; Tilley, T. D.; Majumdar, A.; Neaton, J. B.; Segalman, R. A. Nano. 2011, 5, 9256−9263.

LUMO level, then the current decreases due to the interface tunneling effect in the device, as shown in Figure 9.

4. CONCLUSIONS This article proposed a facile method to fabricate a pyramidshape Alq3 and form a Schottky diode with single pyramidshape Alq3 across Ag electrodes. The diode exhibits rectifying and NDR behavior. These results may be used for organic/ inorganic hybrid nanoelectronics devices.

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AUTHOR INFORMATION

Corresponding Author

*Fax: 886-4-24510182. E-mail: [email protected]. Funding

The authors declare no competing financial interest. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to thank the National Science Council of Republic of China, Taiwan, for financially supporting this research under contract nos. (NSC-100-2112-M-035-002MY3) and (NSC-100-2627-E-035-001). S.-S.L. thanks Dr. D. J. Jan, a member of Physics Division of the Institute of Nuclear Energy Research of Taiwan, for his hospitality and help with FTIR measurements. We are also grateful to Prof. Hoang-Jyh Leu for discussing the microemulsion process.

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REFERENCES

(1) Hebner, T. R.; Wu, C. C.; Marcy, D.; Lu, M. H.; Sturm, J. C. Appl. Phys. Lett. 1998, 72, 519−521. (2) Shaheen, S. E.; Brabec, C. J.; Sariciftci, N. S.; Padinger, F.; Fromherz, T.; Hummelen, J. C. Appl. Phys. Lett. 2001, 78, 841−843. (3) Tang, C.; VanSlyke, S. A. Appl. Phys. Lett. 1987, 51, 913−916. (4) Shi, J.; Tang, C. Appl. Phys. Lett. 1997, 70, 1665−1667. (5) Hung, L.; Tang, C.; Mason, M. Appl. Phys. Lett. 1997, 70, 152− 154. (6) Liu, H.; Li, Y.; Xiao, S.; Gan, H.; Jiu, T.; Li, H.; Jiang, L.; Zhu, D.; Yu, D.; Xiang, B.; et al. J. Am. Chem. Soc. 2003, 125, 10794−10795. (7) Monge, M.; Kahn, M. L.; Maisonnat, A.; Chaudret, B. Angew. Chem., Int. Ed. 2003, 42, 5321−5323. (8) Papadimitrakopoulos, F.; Zhang, X. M.; Thomsen, D. L.; Higginson, K. A. Chem. Mater. 1996, 8, 1363−1365. (9) Chiu, J. J.; Kei, C. C.; Perng, T. P.; Wang, W. S. Adv. Mater. 2003, 15, 1361−1364. (10) Lo, S. S.; Hsu, W. H.; Sie, S. H.; Leu, H. J. J. Mater. Chem. 2012, 22, 12618−12621. (11) Brinkmann, M.; Gadret, G.; Muccini, M.; Talini, C.; Masciocchi, N.; Sironi, A. J. Am. Chem. Soc. 2000, 122, 5147−5157. (12) Collins, A. M.; Olof, S. N.; Mitchels, J. M.; Maan, S. J. Mater. Chem. 2009, 19, 3950−3954. (13) Linda, S. S.; Flocerfida, E. B.; Richard, S. S.; Joseph, L. B.; Krystal, K. C. R.; Daniel, F.; Holger, K.; Kim, F. F.; Paul, E. B. J. Am. Chem. Soc. 2002, 124, 6119−6125. (14) Chen, C. H.; Shi, J. Coord. Chem. Rev. 1998, 171, 161−174. (15) Higginson, K.; Zhang, X. M.; Papadimitrakopoulos, F. Chem. Mater. 1998, 10, 1017−1020. (16) Colle, M.; Gmeiner, J.; Milius, W.; Hillebrecht, H.; Brutting, W. Adv. Funct. Mater. 2003, 13, 108−112. (17) Brutting, W.; Berleb, S.; Muckl, A. G. Org. Electron. 2001, 2, 1− 36. (18) Aviram, A.; Ratner, M. A. Chem. Phys. Lett. 1974, 29, 277−283. (19) Tao, N. J. Nat. Nanotechnol. 2006, 1, 173−181. (20) Pan, J. B.; Zhang, Z. H.; Deng, M. Qiu.; Guo, C. Appl. Phys. Lett. 2011, 98, 013503−013505. 16126

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