Negative thermal quenching of photoluminescence ... - OSA Publishing

Negative thermal quenching of photoluminescence in zinc oxide nanowirecore/graphene-shell complexes S. S. Lin,1,2,3* B. G. Chen,2 W. Xiong,2 Y. Yang,2 H. P. He,4,5 and J. Luo1 1 Department of Information Science and Electronic Engineering, Zhejiang University, Hangzhou 310027, China. State Key Laboratory of Modern Optical Instrumentation, Department of Optical Engineering, Zhejiang University, Hangzhou 310027, China 3 Manchester Center for Mesoscience and Nanotechnology, School of Physics and Astronomy, University of Manchester, Manchester M13 PL, UK 4 State Key Laboratory of Silicon Materials, Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China 5 [email protected] * [email protected]

2

Abstract: Graphene is an atomic thin two-dimensional semimetal whereas ZnO is a direct wide band gap semiconductor with a strong light-emitting ability. In this paper, we report on photoluminescence (PL) of ZnOnanowires (NWs)-core/Graphene-shell heterostructures, which shows a negative thermal quenching (NTQ) behavior both for the near band-edge and deep level emission. The abnormal PL behavior was understood through the charging and discharging processes between ZnO NWs and graphene. The NTQ properties are most possibly induced by the unique rapidly increasing density of states of graphene as a function of Fermi level, which promises a higher quantum tunneling probability between graphene and ZnO at a raised temperature. ©2012 Optical Society of America OCIS codes: (160.4236) Materials; (300.6470) Spectroscopy.

References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

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Received 16 Jul 2012; accepted 10 Aug 2012; published 20 Aug 2012 10 September 2012 / Vol. 20, No. S5 / OPTICS EXPRESS A706

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1. Introduction Graphene, a two-dimensional (2D) semimetal of sp2-hybridized carbon atoms with a honeycomb structure, has enormous extraordinary electronic [1–3], mechanical [4] and thermal properties [5]. As a result, graphene has intrigued a lot of interests in the fields of electronics, materials science and condensed matter physics. The electrons in graphene behave like massless Dirac fermions with mobility routinely up to 1 m2/Vs [6]. However, the main drawback of graphene is its zero band gap, which limits its applications in optical devices. In order to utilize this extraordinary material in light-emitting diodes (LEDs), it is a good choice to construct a heterostructure with direct band gap semiconductors. ZnO, with a band gap of 3.37 eV, is well known as an excellent light emitter, due to its large exciton binding energy of 60 meV [7–9]. It is noteworthy that graphene-semiconductor heterostructure is unique as that the density of states (DOS) of graphene is low compared with conventional 2D electrons with parabolic dispersion and DOS increases rapidly as Fermi level increases. Thus, novel semiconductor devices may be designed, such as field-effect tunneling

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transistor [10]. In fact, there is a growing interest in the growth and characterization of ZnO nanowires (NWs) on graphene substrate [11–14].Ye et al. reported ultraviolet LED consisting of ZnO nanorods and graphene nanoribbons [15]. Hwang et al. reported the enhanced photoluminescence (PL) from ZnO films benefiting from the plasmon excitation in graphene [16]. However, there is no report on ZnO NWs-core/graphene-shell complexes, although this structure allows a pretty large surface contact between ZnO and graphene. Herein, ZnO NWscore/graphene-shell complexes are fabricated through solution method. Transmission electron microscope (TEM) characterizations reveal 2D graphene has been successfully coated onto ZnO NWs and densely surrounded the ZnO NWs forming ZnO NW-core/graphene-shell structure. Normally, in semiconductors, PL intensity generally quenches with increasing temperature, which is called thermal quenching, caused by the increase of nonradiative recombination. PL intensity also suffers from non-thermal quenching induced by change of carrier concentration and surface adsorption etc. For those NW-graphene core shell structure, temperature-dependent PL measurements show abnormal phenomenon, that is, negative thermal quenching (NTQ) behavior. The possible mechanism of this NTQ behavior is proposed through understanding electron charging and discharging of graphene by ZnO under photo-excitation. 2. ZnO NWs-core/graphene-shell complexes Graphene solution was prepared by sonication of highly ordered pyrolytic graphite (HOPG) in the N-methyl-2-pyrrolidone (NMP) solution for 24 hours [17]. The obtained graphene solution was further centrifuged at 10000 rpm for 10 minutes to obtain monolayer graphene. The products are graphene flakes with average size of ~1 µm2. The Quantization effects are negligible. Undoped ZnO NWs were grown by catalyst-free vapor phase transportation method [18] and were subsequently added into the centrifuged graphene solution. The mixed solution was further sonicated for 5 minutes to readily coat ZnO NWs with graphene. Then the mixed solution was dropped onto Lacey carbon grid (obtained from Gatan Ltd.) for morphology characterization using a Philips CM200 TEM operating at 200 kV. Temperaturedependent PL measurements were carried out on a FLS920 fluorescence spectrometer (Edinburgh Instruments) using excitation light of 300 nm produced by a Xe lamp. The sample for PL measurements was obtained through spinning coating the ZnO-graphene solutions on the silicon substrates.

Fig. 1. ZnO NW-core/graphene-shell structure (a-c) Typical low-magnification TEM images show the ZnO NWs were covered with ultrathin graphene layers (d) a high-magnification image of the area as indicated by the arrow in (c) shows all of the ZnO NWs surfaces have been surrounded by graphene.

Figure 1 shows a typical TEM image of ZnO NWs-graphene complexes. We have characterized more than ten NWs, all of which are covered by ultrathin graphene flakes. Most of the fabricated graphene layers are single atomic layers according to the previous report [17]. Figure 1(a)-1(c) show typical ZnO NWs covered with graphene layer. Although some

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areas of ZnO NWs are not obviously covered by graphene layers under low-magnification TEM image, the high magnified TEM image (typically shown in Fig. 1(d)) shows graphene flakes surround ZnO NW along all its length. Figure 1(d) also reveals that distance between ZnO NW and graphene is very small, indicating graphene is in well contacting with the smooth ZnO NW surface. As ZnO NWs are simply mixed with graphene and no further treatment was proceeded, we suggest that the functionalization must be non-convalent and has little influences on the electronic structure of graphene [19]. 3. Results and discussion Figure 2(a) shows the near band-edge PL spectrum of ZnO-core/graphene-shell structure at 14 K, which can be deconvoluted into three peaks, locating at 373.1 nm (A peak, 3.324 eV), 377.3 nm (B peak, 3.287 eV) and 383.6 nm (C peak, 3.233 eV), respectively. The absence of exciton related emission may be due to the screening effect [20] by the 2D electron gases in graphene. Figure 2(b) shows the temperature-dependent PL spectra of ZnO NWcore/graphene-shell structure. As the temperature increases, the intensity of band-edge emission of the NW-graphene complexes increases, in contrast to that of pure ZnO NWs structures [21–23]. Figure 3 shows the temperature dependent PL spectra of pure ZnO NWs, which reveal that both the band-edge and deep level emission decreases monotonously as the temperature increases. The inset of Fig. 3 shows the fine structures of near band-edge PL spectrum at 16K, which is composed of three peaks, i.e., donor bound exciton line at 3.358 eV, defect states-related emission at 3.31 eV [24], and donor-acceptor pair (DAP) recombination at 3.24 eV. The remarkable difference between the ZnO-graphene complexes and pure ZnO NWs reveals that graphene-shell has played a significant role in the emission of ZnO NWs.

Fig. 2. (a) PL spectrum of ZnO NWs-core/graphene-shell structure at 14K, which can be deconvoluted into three peaks. (b) Temperature-dependent near band-edge PL spectra of ZnO NWs-core/graphene-shell structure. Each spectrum is vertically separated for clarity.

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Fig. 3. Temperature-dependent PL spectra of pure ZnO NWs, the inset shows the near bandedge PL spectra of ZnO NWs at 16K.

Figure 4(a) plots the intensity of A, B, and C bands as a function of temperature. It is found that A, B peaks increases dramatically with increasing temperature, inducing the NTQ of the band-edge emissions. A multi-level model of describing NTQ has been developed by Shibata [25]. The PL intensity can be expressed as [25,26]: w

1 + ∑ D q exp(−E 'q / k BT ) I (T ) = I (0)

q =1 m

(1)

1 + ∑C j exp(−E j / k BT ) j =1

where E is the activation energy, k B is the Boltzmann constant and T is the temperature.

E 'q describes the activation energies for processes that increase the PL intensity with increasing temperature while E j is the activation energy for the nonradiative channels. The physics behind the model can be understood as follows. The denominator item in Eq. (1) describes the normal thermal quenching process through nonradiative recombination of photo-excited carriers. However, in some cases the carriers can be captured by trap levels at low temperature. As the temperature increases, the carriers can be released from the trap levels and recombine radiatively, which results in increase of the PL intensity. We fitted the intensity of B peak as a function of temperature using Eq. (1), where E 1' = 30 meV can be deduced. Considering the exciton binding energy of ZnO is 60 meV, the barrier height between graphene and ZnO (Φ B ) should be around 90 meV. Herein, in the core-shell structure, atomic thin graphene serves as an electron reservoir, which accepts electrons thermally emitted from ZnO NWs. The work function of graphene is ~4.5 eV [27] while the electron affinity of ZnO is around 4.35-4.5 eV [28–30]. Thus, Φ B agrees well with the work function discrepancy between graphene and ZnO. Figure 4b also presents the PL intensity of green emission band as a function of temperature ranging from 30 K to 200 K, which shows that the deep level emission also has a NTQ behavior.

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Fig. 4. (a) Temperature-dependent intensity of A, B, C peaks as a function of temperature, the red solid line is the fitting curve using Eq. (2). (b)Temperature dependent defect emission in ZnO-NW core/graphene-shell structure.

It is reported that ZnO nanoparticles can transfer photo-generated electrons to carbon nanotubes, resulting in the quench of ZnO emission [31]. On the other hand, the PL quenching of the green band of ZnO can serve as a probe to monitor the electron transfer from excited ZnO to graphene oxide [32]. However, deduced from the increased defect emission at elevated temperatures (Fig. 4(b)), the charge transfer from the deep level defect in ZnO NWs to graphene is not reasonable, as the transfer will decrease the defect emission as temperature increases. Assuming graphene is n-doped as a 2D electron gas, the energy band diagram between ZnO-graphene interface is schematically drawn in Fig. 5. There is a schottky barrier between ZnO NWs and graphene [15], which results in a higher electron concentration in graphene and the up-bending of ZnO valence band. The valence band-bending of ZnO facilitates the accumulation of holes on the surface of ZnO NWs. The electron flowing from ZnO NWs to graphene should be dominated by thermionic emission mechanism which is described by the following equation:

J = A *T 2e −q ΦB / k B T (e qV / k B T − 1)

(2)

where A* is the Richardson constant, V is the forward voltage induced by the unequal carrier yield coefficient of ZnO and graphene under light excitation. Under photo-excitation, ZnO will produce more electron-hole pairs than graphene, thus V is equal to a positive and electrons flow from ZnO to graphene. As temperature increases, Φ B becomes lower and more photo-excited electrons in ZnO conduction band can transit into graphene and recombine with holes trapped on the surface of ZnO, which partially results in the disappearance of exciton related emission in the core-shell structure, besides the screening effect [20]. As the temperature increases, more electrons tend to reach graphene side and recombine with holes accumulated in ZnO surface, inducing the NTQ behavior. Moreover, graphene has

3 aγ 0 , a is the lattice parameter of 2 2.46 Å, γ 0 is the nearest neighbor hopping energy. The linear dispersion determines the DOS

a linear electronic structure [33]: E = γ k , where γ =

4E . It is obviously that as temperature 2πγ 2 increases, the Fermi level of graphene is lifted up and the corresponding quantum tunneling pathway from graphene to ZnO is increased. Thus, the NTQ should be promoted by the unique DOS dispersion of graphene as more electrons accumulated in the graphene side tend to recombine with holes accumulated at two acceptor levels on the ZnO surface, which results in enhanced intensity of A and B peaks observed experimentally. It is noteworthy that the C band in Fig. 2(a) has almost no change in the temperature range, quite different from the A of graphene is also linearly dispersed: DOS =

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and B bands. The C band is close in energy to the 3.24 eV peak in Fig. 3, suggesting a DAPlike nature. In a DAP complex, electrons are not free but bound to donor impurities. Therefore, the above mechanism can hardly affect its recombination rate. This is also illustrated in Fig. 5.

Fig. 5. Schematic band energy diagram between graphene and ZnO NWs, where Ec, Ev, Ef, ED represents conduction band, valence band, Fermi level and donor level, respectively. The holes assemble on the surface of ZnO NWs due to the up-conversion of valence band, which limits the exciton emission in the ZnO NWs. The recombinations giving rise to A, B emissions can be described as below: The photo-excited electron in ZnO is thermally emitted into graphene part and then the electron recombines with holes accumulated at ZnO surfaces with energy level of A and B. The C emission is a donor-acceptor pair (DAP) recombination.

4. Conclusion In summary, NTQ behavior of PL has been observed in the ZnO NW-core/graphene-shell structure. The mechanism is understood in terms of the charging and discharging effect between 2D atomic thin graphene and ZnO NWs, which results in the recombination of electron in graphene with holes accumulated at the surface of ZnO NWs. The unique linearly DOS dispersion relationship of graphene enhances the NTQ behavior. This research indicates that graphene-ZnO heterostructures may find unique applications in optoelectronic devices such as LEDs [34]. Acknowledgments S. S. LIN should give great thanks to Andre K Geim for the fellowship supporting his research in the University of Manchester. S. S. LIN also thanks the support from Limin Tong. S. S. LIN also thanks to funding support from State Key Laboratory of Modern Optical Instrumentation (111306*A61001) and the China Postdoctoral Science Foundation (20100480083 and 201104714).

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