Cu2O/MgO band alignment and Cu2O-Au ... - OSA Publishing

Cu2O/MgO band alignment and Cu2O-Au nanocomposites with enhanced optical absorption Xuemin Wang,1 Dawei Yan,1 Changle Shen,1 Yuying Wang 1, Weidong Wu,1,* Weihua Li,1 Zhongqian Jiang,1,2 Hongwen Lei,1,3 Minjie Zhou,1 and Yongjian Tang1 1

Science and Technology on Plasma Physics Laboratory, Research Center of Laser Fusion, CAEP, Mianyang, 621900, Sichuan, P. R. China 2 College of Science, Southwest University of Science and Technology, Mianyang, 621010, Sichuan, P. R. China 3 Department of Physics and Key Laboratory for Radiation Physics and Technology of Ministry of Education, Sichuan University, Chengdu, 610064, Sichuan, P. R. China * [email protected]

Abstract: Single crystalline Cu2O film has been successfully synthesized on MgO(100) surface through laser molecular beam epitaxy. In situ reflection high-energy electron diffraction was employed to study the epitaxial growth of Cu2O. The composition and structure of the Cu2O were studied in detail by in situ X-ray photoelectron spectroscopy and transmission electron microscopy. Valence band structures of Cu2O/MgO heterojunction were investigated by in situ X-ray photoelectron spectroscopy and in situ ultraviolet photoemission spectroscopy. The valence band offset was found to be 0.54 eV. By alternative deposition, Cu2O-Au nanocomposites were prepared, which were characterized by in situ reflection high-energy electron diffraction, in situ X-ray photoelectron spectroscopy and transmission electron microscopy. Interestingly, below some critical content of Au, the epitaxial growth of Cu2O recovered after the deposition of Au. Due to the surface plasmon resonance of formed Au colloids, enhanced optical absorption at the wavelength from 600 nm to 800 nm was observed, which is in well agreement with the Mie theory. Depending on the pulses of Au, the position and the width of the absorption peaks can be easily changed. ©2013 Optical Society of America OCIS codes: (160.1890) Optical properties; (220.4000) Microstructure fabrication.

References and links 1. 2. 3. 4. 5. 6. 7. 8. 9.

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Received 30 Sep 2013; revised 20 Oct 2013; accepted 21 Oct 2013; published 29 Oct 2013

(C) 2013 OSA 1 November 2013 | Vol. 3, No. 11 | DOI:10.1364/OME.3.001974 | OPTICAL MATERIALS EXPRESS 1974

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1. Introduction Cuprous oxide (Cu2O) with a band gap of 2 eV has been under intensive investigation for its potential use in hydrogen production under visible light, negative electrode materials for lithium ion batteries, photovoltaic cells, et al [1–4]. Recently, to meet worldwide electricity demand and reduce PV costs as compared with crystalline silicon, special attention has been paid to Cu2O [5,6]. Especially, some work into homojunction and heterojunction solar cells of Cu2O and metal oxides has been made [7–9]. However, the reported highest solar cell conversion efficiency is only 4.08% from a polycrystalline Cu2O, far from the theoretical value of 20% [10,11]. It is well known that the performance of a heterojunction solar cell is greatly affected by the band alignment at the heterointerface. To understand the charge transport at the interface, the relative band alignment between the two different materials is of critical importance. Therefore, it is necessary to measure the valence band offset (VBO) of all kinds of heterojunction based on Cu2O. To our best knowledge, the band alignment of Cu2O/MgO heterojunction has not been reported. Further, except for making a new Cu2O heterojunction, it also needs to control the optical property of Cu2O for its potential optical devices. For this point, one possible way is the synthesis of metal nanoparticles over certain surfaces. Recently, this way has been paid widespread attention due to the possibility to include metal nanoparticle properties into macroscopic 2D and 3D structures [12,13]. These structures can serve as a basis for constructing nanoscale photonic circuits which can carry optical signals and electric currents, the fabrication of subwavelength waveguide components, etc [14–16]. The specialized optical and electronic properties can be due to the well-known localized surface plasmon resonance (SPR) [17–19], which refers to the excitations of a collective oscillation of conduction electrons confined within the nanoparticle volume. The optical properties of metal nanoparticles were theoretically explained by Gustav Mie in 1908. Since then, there has been a significant advance in both theoretical and experimental investigations [20,21]. For instance, by applying of Au and Ag nanoparticles [22–24], plasmonic nanostructures have been constructed in the use of plasmonic-based technologies such as localized surface plasmon resonance (LSRR) sensors, plasmonic solar cells, metal enhanced fluorescence (MEF) displays, treating cancer cells, etc. Due to the SPR, the enhanced UV-vis emission has been achieved [25,26]. However, the fabrication of plasmonic nanostructures with controlled optical property is not very easy. Up to date, the convenient way to modulate UV-vis emission is still limited and needs to be further studied especially for Cu2O. In this paper, single crystalline Cu2O films were obtained on MgO(100) surface by laser molecular beam epitaxy (L-MBE) with O2 pressure of 5 × 10−3 Pa and deposition temperature of 550°C. The band alignment of Cu2O/MgO heterojunction was investigated by in situ X-ray photoelectron spectroscopy (XPS) and ultraviolet photoemission spectroscopy (UPS). The schematic diagram of the band alignment was obtained. The fabrication of Cu2O–Au nanocomposites for modulating UV-vis emission is completed. By changing the laser pulses on Au target, the optical absorption of Cu2O–Au nanocomposites can be easily adjusted.

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2. Experimental details 2.1 Sample preparation Single crystalline Cu2O and Cu2O–Au nanocomposites were deposited on MgO(100) substrate by L-MBE [27]. The schematic diagram of Cu2O–Au nanocomposites was present in Fig. 1. The corresponding experimental parameters were summarized in Table 1. High purity targets (CuO:99.9%, Au:99.999%) were used for the ablation experiments with a pulsed KrF excimer laser, which provided a beam with 248 nm wavelength and 30 ns pulse duration. The laser beam was moved in spirals and with constant velocity across the target area. Before deposition, the vacuum chamber was evacuated down to 2 × 10−7 Pa and MgO(100) substrates were annealed for 30 minutes at 650°C under background vacuum. Then, a continuous flow of O2, 99.999% purity, was introduced and the pressure was set to 5 × 10−3 Pa. The deposition process of Cu2O–Au nanocomposites involved the epitaxial growth of single crystalline Cu2O (60 min), a number of laser pulses on Au target (without O2 and the background vacuum is about 5 × 10−6 Pa) and then the growth of the second Cu2O layer (20 min). During the deposition process, in situ RHEED monitor was performed in the anti-Bray condition using a 25 keV electron beam under a grazing incidence of 1-3° toward the substrate surface. The samples (Cu2O–Au nanocomposites) denoted as Cu2O-Au-x (where x represents the number of laser pulses on Au target).

Fig. 1. A schematic diagram of Cu2O–Au nanocomposites. Table 1. The experimental parameters for Cu2O and Cu2O-Au nanocomposites Background vacuum Target to substrate distance

~2 × 10−7Pa

Deposition temperature

550°C

Laser work parameter

248nm, 1Hz 2-3 J/cm2, for Au deposition(50 pulses Au, 100 pulses Au, 500 pulses Au and 1000 pulses Au for Cu2O-Au-50, Cu2O-Au-100, Cu2O-Au-500 and Cu2O-Au-1000 samples, respectively) 0.5-0.7 J/cm2, for Cu2O deposition

50mm

2.2 Sample characterization The cross section morphology and microstructure of the films were observed by transmission electron microscopy (Tecnai F20, FEI Company Ltd., USA) and samples were prepared by Focused Ion Beam. In situ XPS and UPS measurements were carried out in a UHV chamber (~2 × 10−8 Pa), which was equipped with an unmonochromatized Mg Kα source (1253.6 eV) for XPS and a HeI source (21.2 eV) for UPS, respectively. Optical transmission spectra were obtained with a double beam UV-visible spectrophotometer with a wavelength resolution of 2 nm and an average acquisition time per point of 0.2 s. Measurements were performed in air and the data were automatically corrected by the spectrophotometer to account for the absorbance of the MgO(100) substrate.

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3. Results and discussion 3.1 Growth of single crystalline Cu2O

Fig. 2. Evolution of RHEED patterns during the growth of Cu2O: (a)MgO(100) surface before the deposition of Cu2O; (b) to (d) RHEED patterns of Cu2O after the deposition time of 1 min, 2 min and 5 min; (e)The RHEED pattern of Cu2O after the deposition time of 60 min and the corresponding the Cu2O [1–10] zone-axis pattern

Fig. 3. (a)The cross-sectional TEM image of Cu2O film; (b)The corresponding high-resolution transmission electron microscopy (HRTEM) image and the insets show the FFT images.

To realize a single crystalline Cu2O on MgO(100) surface, experimental parameters were evaluated from a series of experiments. Figure 2 shows the evaluation of RHEED patterns during Cu2O growth. Streaky RHEED patterns indicate the achievement of a clean MgO(100) surface after thermal cleaning (Fig. 2(a)). With the deposition of Cu2O, streak pattern of MgO(100) gradually degenerated into spots (Fig. 2(b) to Fig. 2(d)). It implied that during the initial growth of Cu2O the lattice mismatch (1.2%) induced a lattice strain which resulted in a three-dimensional growth. Figure 2(e) presents the spotty RHEED patterns after the growth of Cu2O. The horizontal and vertical distances of the diffraction spots correspond in real space to a lattice parameter of 4.34 Å, close to the lattice parameter of Cu2O (4.27 Å) [28] in cubic cell, which suggests the formation of Cu2O. According to the miller indices labeled in the schematic diagram, it was found that the film surface was Cu2O(100) plane, which followed an overlap in-plane orientation with MgO substrate. The cross-sectional TEM image (Fig. 3(a)) demonstrates that Cu2O film has a uniform thickness of ~40 nm. Combining the TEM thickness value with the deposition time, we determined the deposition rate as 0.33nm/min in our experiments. In the corresponding high-resolution transmission electron microscopy

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(HRTEM) image (Fig. 3(b)), lattice fringes between 2.27 Å and 2.33 Å from Cu2O(200) and at 2.13 Å from MgO(200) could be found. The little difference of lattice fringes also indicates the good Cu2O overlap epitaxy on MgO substrate. The corresponding fast Fourier transform (FFT) images (insets in Fig. 3(b)) further confirm that Cu2O(100) film follows an overlap epitaxial relationship on MgO(100) substrate. Therefore, from the results of RHEED patterns and TEM, it is believed that single crystalline Cu2O(100) film can be well grown on MgO(100) substrate. 3.2 Valence band structures of Cu2O/MgO heterojunction To further confirm the formation of Cu2O, a film (~20 nm) was measured by in situ XPS and the result was shown in Fig. 4. The insert indicates the Cu 2p core level spectrum. In the Cu 2p core-level region, the Cu 2p3/2 and 2p1/2 peaks are observed at ~933.0 eV and ~953.0 eV, respectively. The weak peaks at the lower binding energy side of the main peaks (marked by black solid rectangles) are the 2p3/2 and 2p1/2 levels excited by satellite lines of the Mg Kα X-ray radiation [29]. The appearance of the shake-up statellites of Cu 2p1/2(marked by a black arrow), which is about 13.0 eV higher than that of the Cu 2p1/2, suggests the formation of Cu2O [30,31].

Fig. 4. In situ XPS survey scan of Cu2O film. The insert indicates the corresponding Cu 2p core level spectrum.

To access the CLs’ (core levels) signals from Cu2O/MgO heterointerface, a thick Cu2O film (~42 nm) after deposition was first thinned by a 3 keV Ar ion sputtering process after it had been loaded into the XPS/UPS analysis chamber. The etch rate was about 0.81 nm/min as calibrated by another Cu2O film. Figure 5 presents in situ XPS spectra recorded during the etching process of the film. Due to the absence of C 1s peak, to compensate for the systematic error in the XPS measurement, the energy position was first adjusted by comparing the binding energy of the O 1s peak to that of the standard binding energy, 530.2 eV [32]. Curve (a) was from the as-grown surface of the sample, on which Cu 3p peak could be clearly observed. The Mg 2p3/2 peak (~50.0 eV) began to appear at a nominal thickness of 1.5 nm as indicated in curve (e). It showed that the CLs of both film and substrate were quite accessible at thickness lower than about 1.5 nm.

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Fig. 5. In situ XPS spectra recorded during the etching process of the Cu2O film: (a)before Ar ion sputtering, (b) to (e)Ar ion sputtering 10min, 20min, 30min and 50min, respectively.

Before Ar ion sputtering, the Cu2O film (~42 nm) was investigated by in situ UPS and XPS, respectively. Figure 6(a) shows the corresponding UPS valence band spectrum. In the binding energy range from −2.0 eV to 8.0 eV, the UPS curve exhibits well defined region. The VBM (valence band maximum) of Cu2O ( EVCu2 O ) was determined by intersection between linear fitting to the background and the leading edge of the spectrum [33]. A value of 2.09 eV was obtained, which is slightly larger than other results [34,35]. These discrepancies can be reasonably due to the difference in growth methods. Figures 6(b) and 6(c) show the Cu 2p core level spectrum by XPS for thick (~42 nm) and nominal thin films (~1.5 nm), respectively. Being a typical heavy metal, spin-orbit splitting in Cu was so large that 2p3/2 and 2p1/2 core levels’ peaks were clearly separated (~20 eV). To precisely determine the peaks’ position, Shirley background and Lorentz-Gauss profiles were taken for the deconvolution. The corresponding binding energies of Cu 2p3/2 were determined to be 932.40 eV and 932.59 eV, respectively. Figure 6(d) shows the Mg 2p3/2 CLs from the nominal thin films (~1.5 nm) and a value of 49.45 eV was obtained. Similarly, the CL of Mg 2p3/2 and VBM position in MgO are determined to be 49.86 eV and 3.23 eV, respectively, as shown in Figs. 6(e) and 6(f). As proposed by Kraut et al [36], core-level alignment can prevent many measurement errors giving an accurate band offsets measurements. According to this method, the valence band offset (VBO) at the Cu2O/MgO interface can be calculated using the following equation Cu 2O Cu 2O MgO MgO EVBO = ECL + ( EMg ) − ( ECu ) 2p3/2 − E V 2p3/2 − E V

(1)

Cu 2 O Cu 2O MgO MgO where ECL = ECu 2p3/2 (i) − E Mg 2p3/2 (i) is the CL offset at the interface. E Mg 2p3/2 and ECu 2p3/2 are

the values of CL energy from bulk MgO and Cu2O, respectively. From Eq. (1), the EVBO can be calculated as 0.54 eV. The conduction-band (CB) offset can be calculated by ΔEC = EgCu2 O − EgMgO − ΔEVBO . The band gaps of Cu2O and MgO are 2.00 eV and 7.83 eV at room temperature, respectively [37,38]. Therefore, the CB offset is estimated to be −5.29 eV. The schematic diagram of the band alignment is shown in Fig. 7. The ratio of conductionband offset (CBO) and VBO ( ΔEC / ΔEVBO ) is estimated to be about 10.

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Fig. 6. (a)Valence band maximum measured by UPS and (b)Core level energy spectrum measured by XPS for Cu2O with nominal thickness of 42 nm. Core level energy spectrum measured by XPS for Cu2O film with nominal thickness of 1.5 nm on MgO(100) surface are indicated in (c) and (d). Core level energy spectrum measured by XPS for MgO(100) are shown in (e) and (f). Dotted curves are original date.

Fig. 7. The schematic band diagram for Cu2O on MgO(100) heterojunction system.

3.3 Growth of Cu2O-Au nanocomposites Figures 8(a)-8(l) show the variation of RHEED patterns during the growth of Cu2O-Au nanocomposites. According to the RHEED patterns along the [ 111 ] azimuth (Figs. 8(a), 8(d), 8(g) and 8(j)), the same crystal structure for the first Cu2O layer was obtained. The corresponding lattice constant is determined to ~4.33 Å for all the samples. After the formation of first Cu2O layer, Au and second Cu2O layer were sequentially deposited. Figures 8(b), 8(e), 8(h) and 8(k) are the RHEED patterns after Au deposition. From these RHEED patterns, it was found that the in-plane lattice of Cu2O increased to 4.42 Å. This change can be ascribed to the size difference between Au ion (1.37 Å) and Cu ion (0.96 Å). In our case, the energy of Au ions produced by laser ablation was assumed from ~40 eV to ~100 eV, and Au ions could penetrate into Cu2O surface from 6.0 Å to 8.0 Å calculated by TRIM code [39,40]. The instantaneous (10−11-10−13 s) high temperature (~103 K) and high pressure (~109 Pa) in local region can be induced by such an implantation of Au ions. Therefore, the replacement of Cu by Au takes place, which results in the increase of the in-plane lattice. As indicated in Figs. 8(c) and 8(f) for Cu2O-Au-50 and Cu2O-Au-100, after the deposition of

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second Cu2O layer, the RHEED patterns recovered. However, for Cu2O-Au-500 and Cu2O-Au-1000 some diffraction spots disappeared. Therefore, it is believed that the epitaxial growth of Cu2O can recover above the Au implanted Cu2O below some critical content of Au.

Fig. 8. The variation of RHEED patterns during the growth of Cu2O-Au nanocomposites. (a), (d), (g) and (j) RHEED patterns after the deposition of the first Cu2O layer for Cu2O-Au-50, Cu2O-Au-100, Cu2O-Au-500 and Cu2O-Au-1000, respectively. (b), (e), (h) and (k) RHEED patterns after the deposition of Au for Cu2O-Au-50, Cu2O-Au-100, Cu2O-Au-500 and Cu2OAu-1000, respectively. (c), (f), (i) and (l) RHEED patterns after the deposition of the second Cu2O layer

Results from TEM studies are shown in Figs. 9(a)-9(d). As the FFT images show (insets in Fig. 9), for Cu2O-Au-50 and Cu2O-Au-100 the first and the second Cu2O layer are crystalline whereas for Cu2O-Au-500 and Cu2O-Au-1000 the second Cu2O layer is not. This result further confirms the above suggestion by RHEED. Further, as in the high-resolution transmission electron microscopy (HRTEM) image (Fig. 10) for Cu2O-Au-100, lattice fringes at 2.13 Å and 2.26 Å from Cu2O(200) and at 2.35 Å from Au(111) could be observed. The lattice fringes of Cu2O confirm the good epitaxial growth of Cu2O. And the lattice fringe of Au indicates that Au colloids form in the crystalline Cu2O.

Fig. 9. TEM images of Cu2O-Au nanocomposites: (a)Cu2O-Au-50, (b)Cu2O-Au-100, (c)Cu2OAu-500, (d)Cu2O-Au-1000. The insets show the corresponding FFT images for MgO substrate, the first Cu2O layer, the Au deposition layer and the second Cu2O layer, respectively.

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Fig. 10. HRTEM image of Cu2O-Au-100.

Cu2O-Au nanocomposites were also investigated by in situ XPS. Figure 11 presents the survey scan of the entire binding energy of 0-1100 eV. Due to the absence of C 1s peak, to compensate for the systematic error in the XPS measurement, the energy position was first adjusted by comparing the binding energy of the O 1s peak to that of the standard binding energy, 530.2eV. As shown in the figure, the peaks of Cu, Au and O exist. With the increasing number of Au pulses, the intensity of Au 4f peaks grows rapidly. Figure 12 shows the Cu 2p and Au 4f high-resolution XPS spectra of different Cu2O-Au nanocomposites. As indicated in Fig. 12(a), two components with Au 4f7/2 binding energies at 84.00 eV and 84.60 eV can be respectively assigned to metal Au and AuCu [41,42]. It is further demonstrated by the change of Cu 2p3/2 binding energies (from 931.60 eV to 932.85 eV) from Fig. 12(b). Therefore, we propose that with the increasing number of Au pulses, metallic Au begins to change into AuCu. Table 2 summarizes the Au:Cu atomic ratio of Cu2O-Au nanocomposites as evaluated by in situ XPS, which shows that Au is gradually enriched in the surface region with the increasing number of Au pulses. Table 2. Au:Cu atomic ratios of Cu2O-Au nanocomposites by in situ XPS

Atomic ratio

Cu2O-Au-50

Cu2O-Au-100

Cu2O-Au-500

Cu2O-Au-1000

1/13.37

1/2.88

1/1.00

1/0.63

Fig. 11. The survey scan of the entire binding energy of 0-1100 eV for Cu2O-Au nanocomposites.

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Fig. 12. Cu 2p and Au 4f high-resolution XPS spectra of Cu2O-Au nanocomposites.

3.3 UV-visible absorption spectra Figure 13 gives the UV-vis absorption spectra of Cu2O-Au nanocomposites. Due to thin thickness of single crystalline Cu2O film, no obvious absorption peak was observed in the range of 500-900 nm. However, for Cu2O-Au nanocomposites, enhanced linear absorption peaks were found. With the increasing number of Au pulses the absorption peaks shift to longer wavelength (from 654 nm to 784 nm) and became widen. The enhanced absorption peaks can be due to the surface plasmon resonance of Au particles [43–45]. In the case of a metal sphere, the dielectric function ( ε eff ) of composite medium can be expressed as

ε eff = ε d + 3 pε d

εm − εd ε m + 2ε d

(2)

where p is the volume fraction of the metal particles in the matrix, ε m and ε d are the dielectric constant of the metal and the matrix, respectively. Furthermore, for colloids with diameters less than λ/20 (λ is the wavelength of the incident light), the absorption can be described by Mie scattering theory in the electric dipole approximation and is given by

α=

  18π nd3  pε 2  λ  (ε1 + 2nd2 )2 + ε 22   

(3)

where α is the absorption coefficient, ε (λ ) = ε1 + iε 2 is the dielectric constant of the metal, nd is the index of refraction of the dielectric host. When ε1 + 2nd2 = 0 , the absorption peak at the surface plasmon resonance frequency is expected to be observed. Obviously, the surface plasmon resonance frequency depends on the electronic properties of the metal colloids and the size of the metal particles. Using the documented values of n(λ ) and k (λ ) , ε (λ ) of Au

can be obtained as shown in Fig. 14. To Cu2O, nd = 2.7. As indicated in Fig. 14, to Cu2O-Au nanocomposites the plasmon resonance absorption peak is determined to be about 695 nm. From Fig. 13, it can be seen that the absorption peak (709 nm) of Cu2O-Au-500 is in well agreement with the calculated value. With the increasing number of Au pulses, the plasmon resonance absorption peak increases (from 668 nm to 778 nm). This suggests that with the increasing of Au particles content the plasmon resonance absorption peak increases.

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Fig. 13. UV-vis absorption spectra of Cu2O-Au nanocomposites.

Fig. 14. The dielectric constant of Au vs. wavelength. The red line is −2 nd , where nd is the refractive index of Cu2O. 2

4. Conclusion

In summary, single crystalline Cu2O film has been successfully deposited on MgO(100) surface by laser molecular beam epitaxy. Based on in situ XPS and UPS measurements, the band alignment of Cu2O/MgO heterostructure were calculated and illustrated. For the first time, Cu2O-Au nanocomposites were prepared by alternative deposition. Depending on the number of Au pulses, Cu2O-Au nanocomposites exhibit different structures of the Au doped layer and the second Cu2O layer. We propose that the surface plasmon resonance of formed Au colloids induces the enhanced optical absorption at the wavelength from 600 nm to 800 nm. By changing the Au pulses, the property of the enhanced optical absorption can be easily controlled. This finding may open a new approach for the fabrication of metal/metal oxide nanocomposites with novel optical absorption property by an easy way. Acknowledgments

This project was financially supported by Ministry Of National Science And Technology Special Major Instrument Program (2011YQ130018). Thanks for the materials tests from Analytical and Testing Center of Southwest University of Science and Technology.

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