Disruption of the Chemical Environment and Electronic Structure in p

Article pubs.acs.org/JPCC

Disruption of the Chemical Environment and Electronic Structure in p-Type Cu2O Films by Alkaline Doping F. Caballero-Briones,*,† A. Palacios-Padrós,‡ O. Calzadilla,∥ I. de P. R. Moreira,‡,§ and Fausto Sanz*,‡,⊥,# †

Laboratorio de Materiales Fotovoltaicos, CICATA-IPN Unidad Altamira, Km 14.5 Carretera Tampico-Puerto Industrial Altamira, 89600 Altamira, México ‡ Department of Physical Chemistry and §IQTCUB, Universitat de Barcelona, Martí i Franquès 1, 08028 Barcelona, Spain ∥ Facultad de Física, Universidad de La Habana, San Lázaro y L, 10400 Vedado, La Habana, Cuba ⊥ Institute for Bioengineering of Catalonia, Baldiri i Reixac 15-21, 08028 Barcelona, Spain # CIBER-BBN, Campus Río Ebro Edificio I+D, Bloque 5, 1a Planta, C/Poeta Mariano Esquillor s/n, 50018 Zaragoza, Spain S Supporting Information *

ABSTRACT: In this work we present an experimental and theoretical study of Cu2O films doped with alkaline ions (Li+, Na+, K+, and Cs+) prepared by Cu anodization. By X-ray photoelectron spectroscopy we determined dopant incorporation as high as 1% for Na+. Three oxygen species were found: O2− ions in the bulk cuprite structure, adsorbed OH− and oxygen in hydroxylated dopant sites. The main effects of the alkaline doping on the optical properties were a reduction in the direct band gap and an approach of the acceptor level edge to the maximum of the valence band. Electrochemical tunneling microscopy experiments confirmed that the valence band maximum energy position is almost invariant. Additional electrochemical impedance, photoelectrochemical activity, and current sensing atomic force microscopy measurements showed an increase of the carrier density and electrical conductivity and a reduction in the photocurrent response with the dopant ion size. Urbach tail parameter analysis suggested additional interaction between copper vacancy derived states and dopant states. From first-principles calculations with the B3LYP hybrid functional on models for the alkaline-doped Cu2O systems we determined that the main effect of the alkaline substitution of copper atoms consists of polarizing the O states, which causes a reduction in the insulating gap and splitting of the density of states just below the Fermi level. The nature of the oxygen−dopant interaction was also calculated: there is a net attractive interaction for Li−O, a slightly repulsive interaction for Na−O, and a net repulsive interaction for K−O and Cs−O. The repulsive interactions between K+ or Cs+ and O cause an accumulation of the dopant at the surface of the crystallites, whereas for Na+ and Li+ the doping ions are more uniformly distributed in the film bulk. It was found that the surface accumulation of K+ and Cs+ hinders vacancy diffusion and therefore blocks film growth, leading to a reduction of roughness and thickness as the ion size increases.

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INTRODUCTION Cuprous oxide (Cu2O) is a p-type semiconductor with relatively well understood electronic properties. Its conductivity has been proved to arise from copper vacancies which create acceptor states within the band gap (Eg)1−3 at energy values of 0.3−0.5 eV above the top of the valence band (VB).4−6 Copper vacancies (VCu's) can reach concentrations up to 1020 cm−3, but the concentration of free holes at room temperature is only about 1018 cm−3 since not all the vacancies are ionized.2 In addition, it is also known that VCu's mediate the transport of copper through the oxide, therefore playing an important role in the formation of Cu2O layers on metallic copper.3,7 As several applications of Cu2O materials associated with their semiconducting nature are important in the field of nonexpensive solar cells, nonvolatile memories, and other devices, substitutional cation doping has been proposed as one of the most promising strategies to modify the Eg, the acceptor states, and the VB population in Cu2O.8 The effects of cation © 2012 American Chemical Society

doping can be related to the Cu2O crystal structure, which consists of two interpenetrated Cu−O networks held together by nonbonding Cu−Cu interactions. From the theoretical point of view, Nolan and Elliot8,9 have pointed out that the reduction or increase in the Eg comes from the interaction of three kinds of phenomena: (i) the size of the dopant cation, which can influence the Cu−Cu interactions in the Cu2O host lattice (e.g., Sn2+ widens the Eg because its large ionic radius increases the Cu−Cu distances), (ii) the alignment of the dopant electronic states with those in the VB or the conduction band (CB) of Cu2O with or without structural distortion9 (e.g., dopants such as In3+ (larger than Cu+) or Al3+ (smaller than Cu+) cause a reduction of the Eg due to the presence of unoccupied 3s states with much lower energies than the Cu 4s orbitals), and (iii) the Received: March 12, 2012 Revised: May 21, 2012 Published: May 24, 2012 13524

dx.doi.org/10.1021/jp3023937 | J. Phys. Chem. C 2012, 116, 13524−13535

The Journal of Physical Chemistry C

Article

introduction of dopant ion states within the Eg (e.g., Ce4+ causes structural distortion in the Cu−Cu network, but the resulting Eg is narrowed by induced defect states). An example that illustrates the need to fulfill all these requirements is provided by the isovalent doping of Cu2O using Ag+ or Au+. In this case, doping produces little structural distortion into the Cu2O lattice, and thus, Eg is not affected. Only small changes in the hole mobility occur because the d10 levels of Au+ and Ag+ are too low in energy to interact significantly with the d10 levels of Cu+. Notice, however, that the variation of the Eg is not always directly related to the conductivity: In3+ and Al3+ doping reduces the Eg but also the conductivity since they bind tightly to VCu's, obstructing the path of vacancy diffusion. With these guidelines, in this work we present an experimental and theoretical study of Cu2O films prepared by Cu anodization doped with alkaline ions, Li+, Na+, K+, and Cs+, and show the cation effects on the structure, chemical bonding, Eg, Urbach disorder parameter, carrier density, flat band potential, and electrical conductivity. Electrochemical tunneling spectroscopy and optical absorption measurements provide information about the effect of the dopant on the band positions, and first-principles calculations on models for the alkaline-doped Cu2O systems provide additional insight to interpret the origin of the trends observed in the measured properties. The present work is organized as follows: after the Introduction, the Experimental Section describes separately the computational details of the calculations used to interpret some experimental data and the preparation and characterization of relevant properties of the materials. Following, the experimental results are described and discussed with the aid of the hybrid density functional periodic electronic structure calculations. Finally, our conclusions are presented.

built from atomic basis sets (atomic orbitals, AOs) optimized for the crystal environment. The AOs are contracted real spherical harmonic Gaussian-type functions (GTFs). Allelectron atomic basis sets of Gaussian functions optimized for the ionic environment are used for all ions except Cs+. For the alkaline atoms we used an 86-511G basis set for K+ (with 3sp and 4sp exponents of 0.498 and 0.169, respectively17), a 511G(1D) basis set for Li+ (from ref 18), and an 8-511G basis set for Na+ (also from ref 22). For Cs+ we used a Hay and Wadt small core pseudopotential19 with a 31G basis set optimized by Prencipe.20 For O2− we used an 8-411G(1D) basis set (with 3sp, 4sp, and d exponents of 0.4567, 0.1843, and 0.5, respectively, derived from ref 21), and for Cu+ ions we used a reoptimized 8-6-4111G(41D) basis derived from previously published work on copper systems22 and which is similar to the approach used in ref 23. For the calculation of Coulomb and exchange integrals, tolerance factors23,24 of 7, 7, 7, 7, and 14 have been adopted to ensure enough accuracy to calculate energy differences smaller than 10−6 hartree. Integration of k-dependent magnitudes has been carried out using a mesh of 300 k points in the first Brillouin zone (chosen according to the Monkhorst−Pack scheme25 with a shrinking factor of eight and two symmetry operators). Calculations have been performed for Cu2O and for realistic models of doped Cu2O phases by substitution of Cu+ ions by alkaline ions. To mimic a low concentration of defects, a 2 × 2 × 2 supercell of the conventional unit cell containing 32 Cu+ ions and 16 O2− ions has been used to construct the ACu31O16 (A = Li, Na, K, or Cs) phases, leading to a defect concentration of A0.0625Cu1.9375O. In all cases, atom positions and cell parameters have been fully optimized. For the defective cell, one Cu atom has been removed in a similar 2 × 2 × 2 supercell to simulate the paramagnetic phase VCuCu31O16. In this case, a ghost atom (VCu) with a Cu basis set for the defective site has been used to describe the charge distribution in this region. Film Preparation and Electrochemical Methods. Alkaline-doped Cu2O (Cu2O:A) films were grown onto polished polycrystalline Cu disks by the electrochemical procedure described elsewhere4 using 0.1 M LiOH, NaOH, KOH, and CsOH aqueous electrolyte solutions. Briefly, the electrochemical procedure involves a series of applied potential steps that correspond to different electrode processes, i.e., (i) a long cathodization routine for alkaline cation loading, reduction of the native oxide, and surface stabilization, (ii) the formation of an OH-adsorbed monolayer, (iii) a Cu+ dissolution stage,7 and (iv) oxide growth. The Supporting Information contains the details of the Cu electrochemistry in the alkaline electrolytes, the potentiostatic routine for film growth, and electrochemical evidence of cation loading during the cathodization step. The film growth, the electrochemical analysis, and the electrochemical impedance spectroscopy (EIS) measurements were performed using a PGSTAT 12 Autolab potentiostat (Ecochemie, The Netherlands) equipped with a frequency analyzer module (FRA) in the three-electrode configuration. A homemade glass cell was used with the copper disk as the working electrode placed at the bottom of the cell; a large surface Pt/Ir wire was employed as the auxiliary electrode, and as a true reference, a Ag/AgCl (SSC) electrode in electric contact with the solution through a Luggin capillary filled with the electrolyte solution was utilized. All the potentials described

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EXPERIMENTAL SECTION Computational Methods. The computational study of the electronic structure of Cu2O and the Li-, Na-, K-, and Cs-doped systems has been performed using a periodic approach and hybrid density functional theory (DFT) based methods as implemented in the CRYSTAL09 program.10 A detailed description of the mathematical formulation and the algorithms in CRYSTAL has been previously published.11 The B3LYP hybrid functional12,13 has been used to calculate the band structure of different models for the undoped and doped Cu2O bulk systems. We have chosen this hybrid DFT approximation to the exchange-correlation functional since we are interested in describing the electronic structure of the ground state and, especially, the nature and the magnitude of the insulating band gap. It has been demonstrated previously that these kinds of hybrid approaches provide a better description of the electronic structure of a wide variety of materials,5,14−16 including many different copper oxides, for which the standard local density approximation (LDA) and generalized gradient approximation (GGA) methods largely underestimate the magnitude of the gap. This improved description provided by the hybrid DFT approaches such as B3LYP has also been shown to overcome the limitations of standard DFT methods to describe the electronic structure and properties of magnetic materials in which the strongly correlated nature of the ground state has been a landmark problem in solid-state physics for many years (for a recent discussion, see ref 16 and references therein). In the present periodic approach, crystalline orbitals are built as linear combinations of Bloch functions, which in turn are 13525



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model was used to calculate the contribution of free electrons to the susceptibility. All the procedures used for the fitting are implemented in the SCOUT 98 program. The experimental data were fitted with a standard deviation below 10−4. Current sensing atomic force microscopy (CAFM) images and I−V curves were obtained in a Dimension 3000 microscope (Veeco Instruments) controlled with a Nanoscope IV electronic system and equipped with a preamplifier module, Extended TUNA. The employed tips were Si tips coated with a conductive Cr/Co film, with a final tip radius of about 30 nm. To ensure proper electrical contact, the optimal cantilever deflection value was set by using a conductive tin-doped indium oxide substrate with known resistance (see the Supporting Information for details). For imaging, the scanning rate was 0.50 Hz. The electrical images and the topographic ones were obtained simultaneously at several bias values; the limits of the applied bias were established when an abrupt increase in the current was observed due to surface film oxidation or reduction, respectively. The I−V curves were registered after location of a fairly conductive zone in the electrical signal image and standing the tip on it. Then the bias was swept at 0.1 Hz from the most negative to the most positive values. At least 10 I−V curves were acquired in the same region to obtain an average response curve (see the Supporting Information). The electrochemical tunneling spectroscopy (ECTS) studies were performed at room temperature using a Molecular Imaging microscope head (Agilent, Santa Clara, CA) controlled by a Dulcinea Nanotec electronics (Nanotec Electrónica S.L., Spain). The STM electrochemical cell was made of Teflon with a 0.30 cm2 area exposed to the electrolyte solution delimitated by an O-ring. A Pt/Ir wire concentric to the sample was used as the auxiliary electrode, and a miniature SSC electrode was used as the true reference. Electrochemical scanning tunneling microscopy (ECSTM) tips were prepared with the procedure described elsewhere.30 The faradic tip current at potentiostatic conditions and far from the surface (used as blank spectra in each electrolyte solution) was typically better than 0.01 nA for spectroscopic recordings. A cyclic voltammogram was acquired in the ECSTM cell before each set of experiments to verify the state of the oxide. The tip was engaged at a typical current set point ranging from 2.0 to 3.0 nA, controlling the sample potential (Us) to be 10 mV more positive than the OCP to have the semiconductor in slight accumulation of holes. The ECTS spectra were acquired after momentary disconnection of the feedback and then application of a potential ramp to the tip, typically at 20 V s−1 in a Utip range from +600 to −600 mV.4 Conductance vs sample potential curves were obtained by differentiating the ECTS spectra after smoothing to minimize noise due to the differentiation algorithm.

herein have been measured using this electrode as the reference. Cyclic voltammetry (CV) curves were used to monitor the electrode state and to identify the working potentials and the electrode processes. The EIS measurements were done by applying an excitation signal of 25 mV amplitude (on the order of kT energy). The impedance versus frequency spectra were acquired at fixed sample potentials. Successive impedance versus potential measurements at different frequencies were performed, and the data are represented as Mott−Schottky (M−S) plots (1/C2 vs Us) to calculate the flat band (FB) potential and estimate the carrier density (NA) and the width of the space charge layer (WSCL) as previously reported in ref 4; the semiconductor type was established from the M−S slope. The photoelectrochemical activity (PEA) of the films was tested in a neutral electrolyte (0.1 M NaNO3) using white light coming from a 150 W illuminator (MI-150, Dolan-Jenner Inc., Boxborough, MA) through an optical fiber. The photocurrent response (Ipc) was recorded at different electrode potentials between the open circuit potential (OCP) and cathodic potentials above the corrosion of the Cu2O films. The experiments were done in the intermittent mode that consists of alternated 90 s intervals of dark−light conditions controlled by a mechanical shutter. Film Characterization. Grazing incidence X-ray diffraction (GIXRD) patterns were obtained using a D500 X-ray diffractometer (Siemens, Germany) with the detector pivoting on the 2θ circle, using unfiltered Cu Kα radiation and an incidence angle of 0.5°. Phases were identified using the PDF database.26 The X-ray photoelectron spectroscopy (XPS) analyses were performed in a PHI 560 ESCA-SAM system (Perkin-Elmer, Waltham, MA) using 1486.6 eV energy Al Kα X-rays. The analyzer was a double-pass cylindrical mirror at a base pressure of 1.3 × 10−7 Pa. The XPS spectra were obtained after 5 min of sputtering with 4 keV Ar+ ions at a current beam of 0.36 μA.cm−2. Survey and multiplex XPS spectra were obtained with scanning steps of 1 and 0.2 eV/step with an interval of 50 ms for survey and multiplex modes, respectively. The spectrometer was calibrated using the Cu 2p3/2 (932.4 eV) and Cu 3p3/2 (74.9 eV) lines. Binding energy calibration was based on C 1s at 284.6 eV. The morphology of the films was studied by atomic force microscopy (AFM) in the intermittent contact mode with a Multimode microscope (Veeco Instruments, Plainview, NY) with Si cantilevers of 35 N/m spring constants. The AFM images were analyzed with the WSxM freeware v.4.0 Develop 8.21.27 The roughness (Rrms) and the average height were obtained from 2 × 2 μm2 AFM images. The diffuse reflectance measurements, from which the thickness, Eg, and Urbach tail parameter were calculated (see the Supporting Information for details on the calculations), were performed in a UV-2101 PC spectrometer (Shimadzu, Japan) with an integrating sphere, using a polished Cu substrate as a reference. The optical properties of the Cu2O thin films in the spectral range of interest, 300−800 nm, were fitted by three types of electronic excitations: band gap transitions, interband transitions from the bulk of the VB into the bulk of the CB, and intraband transitions of the electrons in the CB. These transitions were modeled with standard formulas available in the SCOUT 98 program.28 For the Eg transitions, we used the Tauc−Lorentz model.29 The transitions into the upper half of the CB are represented by harmonic oscillators. The Drude

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RESULTS AND DISCUSSION Electronic Structure Calculations. The description of the electronic ground state of Cu2O by means of the hybrid B3LYP DFT functional corresponds to an insulator with a 2.3 eV Eg, in which the valence states near the Fermi energy are of Cu 3d−O 2p mixed valence nature, suggesting a significant covalency of the Cu−O bond. This covalency is also evident from the Mulliken population analysis of the charge distribution. This analysis provides qualitative numbers for the atomic charges (+0.45e for Cu, −0.90e for O) which largely deviate from the formal +1 and −2 ionic charges. Moreover, the overlap population assigned to the Cu−O bonds is positive and large (0.11e), an indication of the attractive covalent interaction. 13526



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plotted for simplicity. To better appreciate the CB states, the scale in the range 2−5 eV has been multiplied by a factor of 5. The small effect of the alkaline ions to the total DOS suggests that the main effect of the substitutions consists of polarizing the O states, reducing the insulating gap (from 2.3 to 2.1 eV) and splitting the density of states just below the Fermi level, leading to a small peak just on top of the VB. This small peak in the DOS plots starts to split in the Li-doped system and becomes well differentiated in the Na-doped system and less marked for the K and Cs systems. The vertical lines in Figure 2 help to visualize the peak splitting. In the empty states above the minimum of the CB, only small changes in positions and population are detected. From the Mulliken population analysis, the alkaline ions exhibit a charge close to the formal +1 ionic charge (0.89, 0.86, 0.87, and 0.96 for Li, Na, K, and Cs, respectively), suggesting that the A−O interaction in the doped systems is essentially ionic. It is also evident that two different O atoms appear, namely, the bulklike O atoms and the nearest neighbors of a defective site A. On the other hand, the Cu atoms bear the same charge as in the bulk Cu2O. However, the most important differences appear in the overlap analysis: while the value of the Cu−O overlap population remains close to the bulk value, the A−O overlap population values show a qualitatively different nature for Li (+0.005e, slightly attractive), Na (−0.011e, slightly repulsive), K (−0.053e, repulsive), and Cs (−0.080e, repulsive). The nature of the A−O interaction, being repulsive for K and Cs, suggests that the saturation of dopant tends to stabilize in the layers near the surface of the crystallites to reduce this repulsive interaction whereas in the Li- and Na-doped materials a more uniform distribution is expected. This is in line with the trend observed for the calculated substitution energies (Esubs per Cu2O unit) (see Table S1 in the Supporting Information) corresponding to 9.5, 19.6, 32.2, and 48.1 kJ/mol for Li, Na, K, and Cs substitutions, respectively. Regarding the VCuCu31O16 model system, the main feature of the DOS diagram is a new set of occupied states just above the Fermi level at ∼0.7 eV above the bulk VB. This defect is consistent with the vacancy defect (VCu) described by Scanlon et al.5 using a similar theoretical model and a hybrid DFT approach. The calculated defect is also consistent with our previously reported electrochemical tunneling microscopy/ spectroscopy experiments that indicate the presence of conducting states at around 0.5 eV above the maximum of the VB.4 Structural and Chemical Characterization. Figure 3 presents the AFM 2 × 2 um2 images (Figure 3a−d) showing the surface morphology of the alkaline-doped Cu2O films, the calculated height distribution (Figure 3e), and the relation between the ion radius and the calculated thickness, the film roughness, and the average height (Figure 3f). It can be appreciated that the average height increases from Cs- to Lidoped films and the corresponding height distribution widens, indicating that we almost have a single nucleation event in the case of Cs+ while for Li+ the nucleation occurs during the whole film growth period. From Figure 3f it can be seen that the film thickness decreases from ca. 130 nm for Li-doped Cu2O to around 60 nm for Cu2O:Cs, together with a decrease in the average grain height and roughness. In a previous work,7 we have shown that around 50% of the film grows underneath the surface by copper vacancy diffusion and the rest corresponds to either a heterogeneous renucleation of dissolved Cu+ ions to form Cu2O or a surface electrochemical reaction. The strong

Regarding the crystal structure of Cu2O, only one type of nearest and next-nearest neighbor distances is observed for Cu and O atoms. The results are very similar to those reported in ref 24 and are taken as the reference results for the doped systems. Regarding the Cu2O-doped systems, from the band structure of ACu31O16 models, it was observed that the electronic states around the insulating gap are essentially originated from Cu and O states, the contribution of the alkaline ions being negligible. In the present work we focus our discussion on the analysis of the density of states (DOS) plots for simplicity since the band structure diagrams do not add any relevant additional information. Thereafter in Figure 2 only the total DOS is

Figure 1. Calculated density of states of the Cu2O system.

Figure 2. Calculated total density of states of Cu2O and ACu31O16 (A = Li, Na, K, or Cs) models. The y scale in the 2−5 eV range is multiplied by 5 for clearness. Vertical lines are only a guide to the eye.

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Figure 3. AFM 2 × 2 μm2 topographic images of the alkaline-doped Cu2O films prepared with (a) Li, (b) Na, (c) K, and (d) Cs. The z scale is 110 nm for the four images. (e) Height distribution calculated from the AFM images of each sample. (f) Calculated thickness, average height, and Rrms of the films (error bars account for at least five images).

plane. The GIXRD patterns display peaks of neither any of the alkaline oxides (A2O)/hydroxides (AOH) nor any A−Cu−O phase. XPS spectra of the films were acquired to study the composition and chemical bond configuration. Survey spectra of the A-doped Cu2O films showed the presence of the core level peaks of Na 1s (1074 eV), K 1s (298 eV), and Cs 1s (728 eV). Li 1s (55 eV) was detected at the noise level and was not quantified. Table 1 summarizes the film composition correlated

reduction in thickness together with the narrow height distribution in Cu2O:Cs films suggests surface blocking by the incorporated Cs, in agreement with the calculated repulsive interactions in the Cs−O bonds. Figure 4 presents the grazing incidence X-ray diffractograms of the prepared Cu2O:A films in the 2θ range of 20−65°. The

Table 1. Chemical Composition of the Prepared Films Calculated from XPS High-Resolution Spectra dopant

ion radiusa (pm)

Li Na K Cs

68 95 133 169

a

[Cu] (atom %) [O] (atom %) 67.2 68.9 69.7 75.7

32.8 30.2 30.1 24.2

[A] (atom %) ND 0.9 0.126 0.119

Pauling radius of the bare ion. The Cu+ radius corresponds to 96 pm.

with the Pauling radius of each alkaline ion. Sodium has a concentration in the film near 1 atom %, while both potassiumand cesium-doped films incorporate around 0.1 atom % of each ion. The high amount of incorporated sodium may arise from the similarity of ionic radius between Na+ and Cu+ that would enable better Na substitution in the Cu2O lattice. In fact, Na− Cu−O is known to form a ternary system with different stable stoichiometric phases.31 The nature of chemical bonding in the prepared films is analyzed in the high-resolution, intensity-normalized XPS spectra of the Cu 2p3/2 and O 1s core levels shown in Figure 5. Vertical lines indicate the reference binding energy (BE) positions for bulk Cu2O at 932.67 ± 0.4 and 530.4 ± 0.2 eV, respectively.32 In the Cu 2p3/2 region only peaks related to Cu2O at around 932 eV were observed. No evidence of the CuO peak at 933.6 eV or of Cu(OH)2 at 934.9 eV was found. The Cu 2p3/2 peaks of the films (Figure 5a) show a slight shift to lower BE with respect to the reported energy for bulk Cu2O as the ion size increases. The observed BE shift corresponds to a relative change of the Fermi level between 1.0 and 1.4 eV.33 In the O 1s peak region shown in Figure 5b, wide spectra with two maxima are observed, indicating the presence of more than one type of oxygen atom in the films. The intensity-normalized O 1s spectra were deconvoluted after Shirley background

Figure 4. Normalized grazing incidence X-ray diffractograms of the Cu2O:A films. The indicated Miller indices correspond to the Cu2O cuprite reflections (PDF 050667). Peaks marked with an asterisk correspond to the Cu substrate reflections (111) at 43.295° and (200) at 50.431° (PDF 040836).

diffractogram of the Cu2O:Li film presents reflections corresponding to the (110), (111), and (220) planes of the Cu2O cuprite phase. The diffractograms of the Cu2O:Na and Cu2O:K films display peaks corresponding to the (111) and (220) Cu2O planes. The diffractogram of the Cu2O:Cs film shows only a weak reflection attributable to the (111) Cu2O 13528



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Figure 5. (a) Cu 2p3/2 and (b) O 1s core level high-resolution XPS spectra of the Cu2O:A films. The dotted line indicates the reference bulk Cu2O position. The inset shows the spectral deconvolution of the Cu2O:Na in three Gaussian components.

Figure 6. Binding energy of the three types of O 1s peaks fitted from XPS spectra.

K- and Cs-doped films may be attributed to distortion caused by the dopant ion accumulation at the film surface, in agreement with the repulsive interactions established by the calculations for these ions. To summarize, alkaline-doped Cu2O films exhibit three types of oxygen species, namely, oxygen in the cuprite bulk, oxygen from surface-adsorbed hydroxyl species, and oxygen closely associated with the incorporated doping cations. The distribution of these oxygen species is related to the nature of the interactions between the doping cation and the oxygen atoms: net repulsive for Cs−O and K−O, slightly repulsive for Na−O, and attractive for Li−O. Optical Absorption. In this section, the behavior of the optical absorption spectra is analyzed and compared with that of the total calculated DOS to lately evaluate the behavior of the Eg. From the maxima of the calculated DOS, we assign the experimental optical transitions as band-to-band, considering a rigid frame without lattice relaxations after hole−electron generation. For the sake of simplicity, we will not use any correction for the final state after the electronic excitation, assuming the rigid band model/approach for band-to-band electronic transitions. We believe that this qualitative approach is enough for the purpose of this paper. Figure 7a presents the calculated DOS for the Cu2O model system. As mentioned before, the VB states within 2 eV below the Fermi level consist of hybridized Cu 3d and O 2p states, whereas the first nonoccupied states are mainly Cu 3d. The figure inset shows the experimental and simulated absorption spectra of Cu2O, calculated using self-consistent (sc) GW inputs. From the report of Bruneval et al.,35 it was shown that the scGW approach is better to estimate the Eg and the electronic structure of Cu2O than LDA, as has been demonstrated also for the hybrid DFT-based approach used in this work.5,22,24 The spectra show distinct bands marked as EB, EBV, A, and E1 in the 2.0−6.0 eV range. The EB band corresponds to the optical absorption threshold,38 while the EBV and A bands are related to the exciton transitions reported for Cu2O.36 The E1 band corresponds to the transition from the top of the VB to the unoccupied Cu 3d levels in the CB.24 In Figure 7 it can be observed that the energy of the assigned band-to-band transitions from the DOS maxima corresponds fairly well to the experiment. The dispersion in the values of the A transition in the DOS arises because it was assumed that transitions from the maxima at −1.2 and −1.6 eV are both possible. Figure 7b

subtraction. The best fitting was obtained with three Gaussian peaks. For the deconvolution, the peak positions were allowed to vary around the reported positions of O 1s in Cu2O (530.4 eV), LiOH (531.3 eV), NaOH (532.8 eV), and KOH (531.7 eV). The corresponding alkaline oxides were discarded in the fitting because hydroxylated surfaces are expected after alkaline anodization.34 The full width at half-maximum (fwhm) for the O 1s peak directly associated with Cu2O was kept around the same value for all systems to allow comparison, and the fitting quality was assessed by χ2 values on the order of 10−4. The inset in Figure 5b shows the deconvolution of the O 1s spectrum of the Cu2O:Na film in three Gaussian components. The peak at the lowest BE, centered at around 530.0 eV, is assigned to O2− ions in the bulk cuprite structure. The next O 1s peak at higher binding energies is attributed to surface-adsorbed hydroxyl ions.34 The widest peak at around 533.0 eV is associated with oxygen in hydroxylated dopant sites as discussed below. Regarding the crystal structure of optimized ACu31O16 systems from our band structure calculations, the nearest-neighbor A− O distances split over a range of values with a large dispersion compared to the single value observed in bulk Cu2O (see the Supporting Information for the calculated distances). The dispersion increases from Li to Cs given that the width of the O 1s peaks is related to the dopant site. Conversely, the observed dispersion is much smaller for the Cu−O nearest-neighbor distances in ACu31O16 systems compared to bulk Cu2O. The adjusted position of the deconvoluted O 1s peaks for each alkaline-doped film is presented in Figure 6 with respect to the reference energy of O 1s in bulk Cu2O. The vertical error bar is the uncertainty of the energy position. The BE of the O 1s peak attributed to oxygen in the Cu2O matrix is almost constant around 530.1 eV as expected given the low dispersion observed in the calculations for the bulk Cu−O bonding distances. The BE of the O 1s peak associated with the dopant ion hydroxide at the surface increases from 532.5 to 533.5 eV with the increase of the ion size. On the other side, the O 1s peak that is related to adsorbed OH− has a BE of 530.5 eV for Cu2O:Li and Cu2O:Na, whereas for Cu2O:K and Cu2O:Cs the BE increases to 532.2 eV. The observed tendency in the BE shift for the O near the dopant sites is in agreement with the change in the nature of the A−O interaction affected by the doping cation. The BE displacement of the OHads peak for the 13529



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considered, but the reader can refer to Figure 2 for additional details. Some other trends can be observed in Table 2. The optical band gap tends to decrease slightly with the ion size as well as the fundamental absorption edge. This effect can be attributed to the polarization induced by the dopant ions. On the other side, the exciton transitions (EBV and A) are affected by the specific dispersion of the DOS below the Fermi level for each dopant ion. Finally, the energy variation of the transitions from the top of the VB to the unoccupied Cu 3d levels corresponds to the observed changes in the CB population. From the optical absorption spectra, the Tauc plots for direct transitions to estimate the Eg were obtained. Figure 8a shows the Tauc plots of the four Cu2O:A films where two direct optical transitions can be observed: one at 2.05−2.10 eV that corresponds to the direct Cu2O band gap (VB−CB)37 and another at 1.85−1.95 eV (depicted in the figure inset) which is related to the transitions from the acceptor level within the Eg to the conduction band (EA−CB).38 The experimental Eg values agree with the band structure calculations, demonstrating the feasibility of our calculation approach for Cu2O. The calculated energies for both transitions are presented in Figure 8b plotted against the ion size. This figure gives evidence of a small reduction of the Eg energy from 2.08 to 2.03 eV with the ion size increase. At the same time, the EA−CB transition energy augments from 1.84 eV for the Li-doped film to an almost constant value around 1.90 eV for the rest of the samples. The decrease of Eg is in accordance to the calculated band gap reduction due to the polarization of the O states upon alkaline doping. On the other side, the variation of the EA−CB gap for the Li-doped film suggests a strong interaction between the acceptor state introduced by the copper vacancies and the strongly polarized Li defects. The nature of such an interaction could be elucidated with a model system, VCuLiCu30O16, with all the possible VCu−Li configurations, but this study is beyond the scope of the present work because of its complexity and computational cost. Electronic Properties at the Electrode|Electrolyte Interface. The ECSTM/ECTS experiments provide a direct probe to explore the electronic states around the Fermi level (EF) within the range of electrochemical stability of the tip.4,39,40,42 ECSTM images are only obtainable when available states for electron tunneling exist. A mapping of the available states can be done at different biases, which in our case can be achieved by a different electrode potential or tip potential against a reference electrode. On the other side, the ECTS experiment consists of a fast scan of the tip potential to do a measurement of the electronic transfer between the tip and the semiconductor states. The derivative plot gives a measure of the density of states available for tunneling, as the electrochemical scale of potentials can be referred to the absolute energy scale through the material’s work function. Details of the technique and its application to different systems are extensively described in our previous works.4,39,40 Particularly, in ref 4 we constructed an electronic diagram of the Cu|Cu2O|electrolyte system when the Cu|Cu2O was immersed in a 0.1 M NaOH electrolyte. A scheme of the electronic diagram is presented in Figure 9. The positions of the VB edge as well as the bottom of the acceptor level (here denoted as SB) were found by ECTS. The optical band gap, discussed above, allowed calculation of the CB edge. The flat band potential (UFB) was determined by EIS; see below. The sign of the tip current in the I−V curve corresponds

Figure 7. Calculated density of states of the model systems (a) undoped Cu2O and (b) Cu2O:K using a simple rigid band model approach. The transitions marked with double arrows were assigned to the calculated Eg and to the EB, EBV, A, and E1 peaks corresponding to the optical transitions indicated in the insets. The inset in (a) (data kindly provided by F. Bruneval, ref 35) shows the experimental (red line) and calculated optical absorption of Cu2O using the BSE solution obtained with scGW inputs (black line). The inset in (b) shows the experimental absorption spectrum of the Cu2O:K film from this work.

presents the total calculated DOS and the experimental optical absorption spectra of the Cu2O:K film (inset). The absorption spectrum consists of broad bands, and the band positions are shifted with respect to the reported Cu2O spectrum. Around each maximum in the DOS, the band splitting discussed before introduces different peaks that could explain the observed band broadening in the experimental spectra. Table 2 presents the derived transition energies from the calculated DOS for the Cu2O:A films. The energies were estimated from the peaks where the maximum DOS is observed, i.e., the first unoccupied state for the CB, the first maximum in the CB Cu 3d states, the maxima of the two peaks between 0 and −1 eV, and the maximum of the peak between −1.5 and −2 eV. The dispersion due to peak splitting is not Table 2. Energy Values (eV) of the Transitions Estimated from DOS Plots Shown in Figure 2

Eg EB EBV A E1

Cu2O

Cu2O:Li

Cu2O:Na

Cu2O:K

Cu2O:Cs

2.3 2.6 2.9 3.9 4.0

2.3 2.6 2.8 3.9 3.8

2.2 2.6 2.9 3.9 3.7

2.2 2.5 2.8 4.1 4.1

2.1 2.4 2.7 3.8 4.0 13530



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Figure 8. (a) Tauc plots of the Cu2O:A films. The inset shows the transition from the acceptor level (EA) to the conduction band. (b) Evolution of the values of the Eg−CB and EA−CB transitions versus the alkaline ion size.

obtain Itip vs Utip curves, and dItip/dUtip was calculated numerically. Figure 10a presents Utip against the absolute value of dItip/dUtip. From this plot, the separation between the VB maximum and the acceptor band minimum was measured. The inset of this graph is an estimation of the error in the determination of the band positions; as error criteria, we set a | dItip/dUtip| > 0.005 to consider the onset of conduction and there the position of the band edges. Figure 10b shows the evolution in the band positions with respect to the OCP of the Cu|Cu2O|electrolyte system. The OCP describes the equilibrium between the electrolyte redox potential and the Fermi level of the semiconductor at the electrode/electrolyte interface. This OCP can be changed by the presence of different surface states, defects, and so on.41 In Figure 10b it is evident that the VB edge position with respect to the OCP of each system is almost constant for all the doped films. On the other side, the EA edge approaches the VB maximum as the ion size increases. We believe that the electrostatic interactions between both types of defects (i.e., the VCu and the alkaline dopant) could explain this observation, although the calculation of such interactions is beyond the scope of this work, as mentioned before. However, an important consequence of the observed reduction of the energy difference between the EA

Figure 9. Scheme of the electronic diagram of the Cu|Cu2O|electrolyte system described in ref 4.

to either hole extraction from the VB or electron injection to the acceptor state. The hole/electron states of Cu2O:A films were investigated by electrochemical tunneling spectroscopy to assess the relative VB−EA positions. The experiments were done at slight depletion conditions (Ue set at 20 mV more negative than the OCP in the corresponding electrolyte, i.e., 0.1 M LiOH, NaOH, KOH, or CsOH). The tip potential (Utip) was swept to

Figure 10. (a) Tip potential (with respect to the OCP) versus the derivative of the tip tunneling current with respect to the tip potential plotted. The inset depicts the derivative value of Itip versus Utip for a blank measure as a measure of the error in the band position. (b) Estimated positions of the VB maxima and the EA edges with respect to the OCP for each Cu2O:A|AOH electrolyte interface. 13531



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Figure 11. (a) Mott−Schottky plots of the Cu2O:A films obtained in a 0.1 M NaNO3 electrolyte (pH 7). (b) Calculated values of WSCL and NA.

edge and VB maximum is that the increase of the EA−CB band gap is now understandable in terms of the previous discussion. Thus, the effect of the alkaline doping on the electronic structure could be summarized as follows: the polarization induced by the alkaline dopant shifts the CB minimum toward lower energies, thus reducing the direct band gap. The electrostatic interactions between the copper vacancy states and the alkaline defects led to a shift of the acceptor level edge toward the maximum of the VB that in turn is not very affected by the alkaline doping. The splitting of the states below the VB maximum originates the observed changes in the absorption spectrum both in the transition energies and in the broadening of the absorption bands. Additional information about the electronic states of semiconducting anodic thin film oxides can be obtained from the electrochemical impedance measurements.42 The impedance versus electrode potential measurements were obtained at a frequency of 40 Hz, where the Bode plots display the maximum phase shift, corresponding to a maximum capacitative behavior.4,42 The Bode plots (not shown) were fitted with an R−RQ circuit, where Q is the so-called constant phase element, characteristic of semiconducting electrodes with a high number of defects. The fitting values allowed the calculation of the film capacitance. The M−S plots 1/C2 vs Ue are presented in Figure 11a; the negative slope indicates p-type conduction for all the films. From the extrapolation to 1/C2 = 0, we obtain the flat band potential (UFB), and Figure 11a shows that its value shifts toward negative values as the ion size increases. Figure 11b presents the estimated values of the carrier density (NA) and the width of the space charge layer (WSCL). The width of the space charge layer is reduced as the carrier density increases as the ion size also increases. The interpretation of the reduced values of the WSCL for Cu2O:K and Cu2O:Cs is not clear, but it could be attributed to Cs accumulation at the film surface that creates a charged layer at the grain boundaries while bulk Cu2O keeps its semiconductor behavior, as suggested by the CAFM measurements discussed below. The PEA of the films was assessed to obtain complementary information on the processes at the electrode|electrolyte interface. The comparison of the PEA for the Cu2O:Li and Cu2O:Cs films is presented in Figure 12. In the case of Lidoped films we observe an increase of the electrode electrical current upon illumination when the potential moves to more negative values, confirming the p-type character of the material. In the case of Na and K, the behavior is the same but absolute current values are lower. In the inset is presented the behavior

Figure 12. Photoelectrochemical activity of the Cu2O:Li and Cu2O:Cs (inset) films in the intermittent illumination mode in a 0.1 M NaNO3 electrolyte.

of the Cs-doped film with the lowest PEA, but still a slight increment when we shift to more cathodic potentials. The PEA response is coherent with the variation in WSCL, allowing a better charge separation in the case of Cu2O:Li films. The values of the photocurrent intensities are summarized in Table 3. Electrical Properties. The doped films were characterized by CAFM. In CAFM, a topographic image and an electrical map are recorded simultaneously under the application of a bias potential, allowing the comparison of zones of different conductivities with the topography. Figure 13 displays the electrical images obtained for the Cu2O:Na film superimposed to the corresponding topographic image in a 500 × 500 nm2 area for two different applied bias potentials, i.e., −1000 mV (Figure 13a) and +1000 mV (Figure 13b). In Figure 13a it is noticeable that the current arises mainly from the grain boundaries and the grain sides when a negative bias is applied, while upon positive potential, the electrical signal is observed from the entire grain surfaces. The higher electrical signal at positive bias compared with the negative bias is consequent with the p-type semiconducting nature of the films, because holes will be accumulating in the surface available for conduction. The conduction at negative bias is expected to come from minority carriers originated in the defects. Figure 14 presents the I−V curves obtained by applying a positive bias to the films, fixing the tip on a region with good conduction. The exponential region of each curve was fitted 13532



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Table 3. Summary of Film Properties Obtained from EIS, AFM, PEA, and UV−Vis Measurements dopant +

Li Na+ K+ Cs+

thickness (nm)

Rrms (nm)

E0 (meV)

132 129 105 58

18 14 13 7

428 215 145 178

NA (cm−3)

UFB (mV)

WSCL (nm)

Ipc at Ue = −425 mV (nA)

× × × ×

−133 −133 −145 −150

5.1 4.4 2.5 1.2

4035 280 111 ND

6.8 8.0 1.4 3.0

1017 1017 1018 1018

film surface, then hindering vacancy migration and film growth. The same phenomenon could be responsible for the variation of UFB, as K+ and Cs+ accumulate on the surface instead of incorporating into the film bulk as Li+ and Na+ do. Regarding the carrier density, the low value of NA in the Li-doped film could be a consequence of the strong polarization and interaction with the VCu and possible neutralization of ionized vacancies, not described by our theoretical model but reported for Cu2O.3 Conversely, the proximity of the VCu acceptor levels to the maximum of the VB in Cs-doped films could increase the number of charge carriers. The Urbach tail parameter, E0, is in fact a sum of contributions that depend on the number of carriers, the temperature, and the structural disorder:43,44

Figure 13. Overimposed topographic/electrical images obtained by CAFM in the Cu2O:Na film, when the sample is biased at (a) −1000 mV and (b) +1000 mV. The z scale in electrical images is 10 nA, and the z scale in topographical images is 60 nm. The z scale in (a) is multiplied by −1 to see the current as bright spots.

E0(n , T , X ) = E0,impurities(NA , ND) + E0,photons(T ) + E0,disorder(X )

(1)

where E0(n,T,X) represents the measured band tail parameter given by the interactions between carriers and both acceptor and donor impurities (E0,impurities(NA,ND)) and between the carrier and phonons (E0,phonons(T)), which incorporates the three terms described in ref 44. The E0,disorder(X) term in eq 1 represents the contribution from the structural disorder given by the sum of the grain boundary contribution (E0,GB), the contribution from the bulk defects (E0,def), including the doping atoms, and the contribution from the structural disorder, i.e., the strain (E0Y). In the case of the studied Cu2O:A films, the decrease in the measured E0 with the cation size is not straightforwardly understandable as in the case of CdTe films that mainly depends on E0,GB,7,8 but it has to be modeled in terms of opposite tendencies, i.e., the reduction in crystallinity observed in XRD versus the reduction in roughness, the increase in charge carriers vs the decrease in grain size, and so on. At a first glance, E0 reduction with respect to Rrms suggests that scattering could be a major contribution to the disorder, although these effects and the dopant−carrier interactions have to be considered in a future paper.

Figure 14. I−V curves obtained by CAFM in the Cu2O:A films in the forward bias region. The inset shows a scheme of the reported Cu2O electronic diagram.

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considering a Schottky behavior. Enhanced conductivity together with a shift of the onset of the forward current toward reduced bias is evident when the dopant size increases. The observed increase in the film conductivity can be related to the increase of charge carriers and the reduction of the space charge layer width calculated from the Mott−Schottky plots. Summary of Selected Properties. Table 3 shows a summary of the measured properties of the Cu2O:A films. From the atomic force images the rms roughess (Rrms) was estimated; from the optical absorption measurements, the thickness and the Urbach tail parameter, E0, were calculated. From the EIS measurements the value of the flat band potential, UFB, and estimated values of the carrier density, NA, and the width of the space charge layer, WSCL, were obtained, and from the photoelectrochemical response, the photocurrent amplitude at Ue = −425 mV was derived. The reduction in thickness and Rrms can be explained by the aforementioned tendency of K+ and Cs+ to accumulate onto the

CONCLUSIONS In this work we present an experimental and theoretical study of Cu2O films doped with alkaline ions (A+ = Li+, Na+, K+, and Cs+). The films were prepared by Cu anodization in 0.1 M alkaline hydroxide electrolyte solutions using a recently developed potential application routine. The as-grown film properties were studied by a set of ex situ techniques: atomic force microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, optical absorption, and current sensing atomic force microscopy/spectroscopy. In situ measurements were also performed, such as electrochemical scanning tunneling microscopy/spectroscopy, electrochemical impedance spectroscopy, and photocurrent response. Indeed, band structure calculations with the B3LYP hybrid functional were done on models for the undoped and the alkaline-doped Cu2O systems. The results of the calculations 13533



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acknowledged for technical support in the ECTS measurements. Financial support from CONACYT (México) through Contract 151675Q and MICINN (Spain) through Project CTQ2007-68101-C02-01 is granted.

showed that the main effect of the Cu substitution by alkaline atoms consists of polarizing the O states and therefore reducing the insulating gap and splitting the density of states just below the Fermi level. The nature of the oxygen−dopant interaction was found to be net attractive for Li−O, slightly repulsive for Na−O, and net repulsive for K−O and Cs−O. The repulsive interactions between K or Cs cations and O cause the dopant accumulation at the surface of the crystallites, whereas for Na+ and Li+ the doping ions are more uniformly distributed in the film bulk. The surface accumulation of K+ and Cs+ hinders vacancy diffusion and therefore blocks film growth, leading to a roughness and thickness reduction for these large ions. The increasing structural distortion with the dopant size increase can also explain the observed reduction in the diffraction peaks. XPS results indicated dopant incorporation as high as 1% for Na and the observation of three different oxygen species in the films that correspond to O2− ions in the bulk cuprite structure and adsorbed hydroxyl ions and oxygen in hydroxylated dopant sites. Reduction of both the direct band gap and the energy gap between the acceptor level edge and the maximum of the valence band were confirmed by the tendencies in the results of the electronic structure calculations. Electrochemical tunneling spectroscopy experiments confirmed that the valence band maximum energy position is almost invariant and that the copper vacancy derived states shift toward this valence band maximum with the increase in the ion size. Urbach tail parameter analysis suggested additional interaction between copper vacancy derived states and dopant states, which could explain the observed shift in the VCu level edge. Electrochemical impedance, photoelectrochemical activity, and current sensing atomic force microscopy measurements showed an increase of the carrier density and electrical conductivity and a reduction in the photocurrent response with the dopant ion size, consistently with the valence band splitting below the valence band maximum.

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(1) Nolan, M; Elliot, S. D. Phys. Chem. Chem. Phys. 2006, 8, 5350− 5358. (2) Raebiger, H.; Lany, S.; Zunger, A. Phys. Rev. B 2007, 76, 045209. (3) Wright, A. F.; Nelson, J. S. J. Appl. Phys. 2002, 92, 5849−5851. (4) Caballero-Briones, F.; Artés, J. M.; Díez-Pérez, I.; Gorostiza, P.; Sanz, F. J. Phys. Chem. C 2009, 113, 1028−1036. (5) Scanlon, D. O.; Morgan, B. J.; Watson, G. W. Phys. Rev. Lett. 2009, 103, 096405. (6) Rakhshani, A. E. J. Appl. Phys. 1991, 69, 2365−2369. (7) Caballero-Briones, F.; Palacios-Padrós, A.; Calzadilla, O.; Sanz, F. Electrochim. Acta 2010, 55, 4353−4358. (8) Nolan, M.; Elliott, S. D. Chem. Mater. 2008, 20, 5522−5531. (9) Nolan, M.; Elliott, S. D. Thin Solid Films 2008, 516, 1468−1472. (10) Dovesi, R.; Saunders, V. R.; Roetti, C.; Orlando, R.; ZicovichWilson, C. M.; Pascale, F.; Civalleri, B.; Doll, K.; Harrison, N. M.; Bush, I.; D’Arco, Ph. Llunell, M. CRYSTAL2009 User’s Manual; University of Torino: Torino, Italy, 2010. (11) Dovesi, R.; Orlando, R.; Roetti, C.; Pisani, C.; Saunders, V. R. Phys. Status Solidi B 2000, 217, 63−88 and references therein. (12) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (13) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785. (14) Hu, J. P.; Payne, D. J.; Edgell, R. G.; Glans, P.-A.; Learmonth, T.; Smith, K. E.; Guo, J.; Harrison, N. M. Phys. Rev. Lett. B 2008, 77, 155115. (15) Muscat, J.; Wander, A.; Harrison, N. M. Chem. Phys. Lett. 2001, 342, 397. (16) Rivero, P.; Moreira, I. d. P. R.; Illas, F. Phys. Rev. B 2010, 81, 205123. (17) Dovesi, R.; Roetti, C.; Freyria-Fava, C.; Prencipe, M.; Saunders, V. R. Chem. Phys. 1991, 156, 11−19. (18) Merawa, M.; Labeguerie, P.; Ugliengo, P.; Doll, K.; Dovesi, R. Chem. Phys. Let 2004, 387, 453−459. (19) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299. (20) Prencipe, M. Laurea Thesis, University of Torino, 1990; pp 91− 92. (21) Towler, M. D.; Allan, N. L.; Harrison, N. M.; Saunders, V. R.; Mackrodt, W. C.; Apra, E. Phys. Rev. B 1994, 50, 5041−5054. (22) Doll, K.; Harrison, N. M. Chem. Phys. Lett. 2000, 317, 282−289. (23) Dovesi, R.; Pisani, C.; Roetti, C.; Saunders, V. R. Phys. Rev. B 1983, 28, 5781−5792. (24) Saunders, V. R.; Freyria-Fava, C.; Dovesi, R.; Salasco, L.; Roetti, C. Mol. Phys. 1992, 77, 629−665. (25) Monkhorst, H. J.; Pack, J. D. Phys. Rev. 1976, 13, 5188−5192. (26) Powder Diffraction Files-2 Data Base; International Centre for Diffraction Data: Newtown Square, PA, 2000; PDF Cards 050667 (Cu2O) and 040836 (Cu). (27) Horcas, I.; Fernandez, R.; Gomez-Rodriguez, J. M.; Colchero, J.; Gomez-Herrero, J.; Baro, A. M. Rev. Sci. Instrum. 2007, 78, 013705. (28) Theiss, M. Hardware and Software for Optical SpectroscopySCOUT 98; Aachen, Germany, www.mtheiss.com (accessed Apr 19, 2011). (29) Jellison, G. E., Jr. Thin Solid Films 1998, 33, 313. (30) Díez-Pérez, I.; Güell, A. G.; Sanz, F.; Gorostiza, P. Anal. Chem. 2006, 78, 7325. (31) Kulakov, A. B.; Maljuk, A. N.; Sofin, M.; Lin, C. T.; Keimer, B.; Jansen, M. J. Solid State Chem. 2004, 177, 3274−3280. (32) NIST X-ray Photoelectron Spectroscopy Database, NIST Standard Reference Database 20, version 3.5, data compiled and evaluated by Wagner, C. D.; Naumkin, A. V.; Kraut-Vass, A.; Allison, J. W.; Powell, C. J.; Rumble, J. R., Jr.; http://srdata.nist.gov/xps/Default. aspx (accessed May 25, 2011).

ASSOCIATED CONTENT

S Supporting Information *

Copper electrochemistry in alkaline media and electrochemical routine for film preparation, reflectance spectra (determination of the optoelectronic parameters), Urbach tail parameter analysis, electrical measurements by current sensing atomic force microscopy, calculated distances between neighbors in the model systems, and calculated substitution energies. This material is available free of charge via the Internet at http:// pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (F.C.-B.); [email protected] (F.S.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS F.C.-B. thanks the Physical Chemistry Department at the Universitat de Barcelona (UB) for use of its research facilities. O.C. thanks the UB for a Visiting Professor Grant. A.P.-P. thanks MEC for financial support through an FPU grant. Technical support from the SCT-UB (Molecular Spectroscopy, Nanometric Techniques, and XRD units) is recognized. Dr. J. L. Peña and Dr. P. Bartolo-Pérez at CINVESTAV-Mérida are acknowledged for the XPS measurements. J. M. Artés is 13534



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