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5, 183

Electronic transitions of the transparentdelafossite-type CuGa1�xCrxO2 system:first-principles calculations andtemperature-dependent spectral experiments

Junyong Wang, Peng Zhang, Qinglin Deng, Kai Jiang, Jinzhong Zhang, Zhigao Hu*and Junhao Chu

The structure and optical properties of the CuGa1�xCrxO2 (CGCOx) system with 0 r x r 1 have been

investigated by combining theoretical calculations and optical experiments. Density functional theory

within the generalized gradient approximation (GGA) was utilized to calculate the electronic structure of

the CGCOx system. It reveals strong hybridization between the 3d states of the transition metal ions and

2p states of the O element, which has an important effect on the electronic transitions of CGCOx

materials. Moreover, to confirm the theoretical results, CGCOx films with different Cr compositions were

deposited via a sol–gel method and the optical properties were measured directly by temperature

dependent UV-Vis transmittance and infrared reflectance spectroscopy. The frequency of two acoustic

modes (Eu and A2u) gradually increases, whereas the values of the electronic band gap decrease linearly

with increasing Cr composition, which can be attributed to the stronger Cr–O covalent interaction.

Remarkably, an additional direct electronic band gap has been observed for the CuGa0.75Cr0.25O2 film,

which shows an abnormal behavior in a low temperature region. It can be assigned to the p–d electron

hybridization at the top of the valence band. These results show that the first-principles calculations

agree well with the experimental data and can be used to explain the microscopic origin of the inter-

band transitions for CGCOx films. The present work further improves the potential applications of

delafossite-type oxides in the field of optoelectronic devices.

1 Introduction

Transparent conductive oxides (TCOs), which combine advantageousproperties such as optical transparency and electrical conductivity,are widely used in optoelectronic fields, such as detectors, flatpanel displays and solar cells.1–3 It is hoped that such trans-parent conductive oxides may be used in the fabrication offunctional p–n junctions, making a variety of transparentelectronic devices possible. However, the currently availableTCOs, such as In2O3:Sn,4 TiO2,5 and ZnO:Al,6 are mostly n-typesemiconductors. Different from these n-type TCOs, p-type TCOsare more elusive and desirable. A series of relevant theoreticaland experimental results suggest that reproducible p-type bulkdoping is difficult to implement,7,8 which can be attributed tothe large electronegativity of oxygen. The valence-band edge ofwide-band-gap oxides is strongly dominated by O ion states.

Moreover, oxidizing oxygen is not easily realized using chemicalmethods, which makes the introduction of positive holes intothese bands more difficult.9 These limitations become an obstaclefor TCO fields and stimulate the development of p-type TCOs.

Effective transport of positive holes requires disperse electronicstates at the top of the valence band. Coincidentally, Cu and Ag,which have complete low-binding-energy d shells, can lead to amore disperse band state. It is attributed to the suitable covalentstates, which are generated from coupling d shells states withthe electronic states of oxygen. The most common p-type oxidebased on Cu is Cu2O but the band gap is too small to serve as aTCO. Therefore, Hosono and co-workers came up with a generalprinciple to design Cu(I) p-type TCOs, based on reduced dimen-sionality in the interaction between adjacent Cu(I) sites com-pared to Cu2O. The first compound developed in this way wasthe delafossite oxide CuAlO2,10 later followed by CuGaO2.11 Thehybridization of Cu 3d states in close energy proximity with O 2penergy levels increases the energy of the valence band maximum(VBM) and delocalizes the hole state to form an intrinsic p-typesemiconductor.12 The CuMO2 delafossite family, where M = Al,Ga, In or Cr, etc., is attracting more attention for the stable and

Technical Center for Multifunctional Magneto-Optical Spectroscopy (ECNU),

Shanghai Department of Electronic Engineering, East China Normal University,

Shanghai 200241, China. E-mail: zghu@ee.ecnu.edu.cn; Fax: +86-21-54345119;

Tel: +86-21-54345150

Received 19th October 2016,Accepted 29th November 2016

DOI: 10.1039/c6tc04535a

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prototype p-type wide band. In the delafossite structure, eachCu atom is linearly coordinated between two oxygen atoms,forming O–Cu–O dumbbells parallel to the c-axis. Oxygens inthese O–Cu–O units are also coordinated to three M atoms andM-centered octahedra form MO2 layers, which lie parallel to theab plane. The excellent optical and electronic properties can beattributed to the O–Cu–O–M interactions. A. B. Bocarsly et al.have reported that the p-type CuRhO2 and CuFeO2 could be excellentphotocathodes for water and CO2 reduction, respectively.13,14

Furthermore, Mg-doping has a significant impact on the con-ductivity and photoelectrochemical activity of CuFeO2. Hence,one can optimize the material and device performances bysubstitution of the host cation, which has been the focus ofrecent research.

Recently, CuGaO2 and CuCrO2 have received great attentiondue to their tolerable transparency and conductivity, which areimportant for the photocatalyst and hole transport layer inphotovoltaic devices.15–17 CuCrO2 sol–gel film electrodes, whichhave an obvious photoelectrochemical behavior, have been pre-pared on conducting glass.18 The performances of CuGaO2 basedp-type dye-sensitized solar cells were improved by Mg-doping.19

The sol–gel derived CGCOx films have been shown to possess arelatively high transmittance and the values of the direct band gapdecrease with increasing x.20 Therefore, a CGCOx system shouldbe fabricated to achieve better properties for wider applications.Moreover, numerous computational studies have also beencarried out to investigate the optical and electronic propertiesof CuGaO2 and CuCrO2. For CuCrO2, the effects of Cu vacancyand the Mg dopant on the band structure have been investigatedusing density functional theory with different exchange func-tionals.12,21 It was found that the mixing of Cr 3d states withO 2p states in the valence band produces shallower transitionlevels for the Cu-based holes. The electronic and optical propertiesof CuGaO2 have been calculated.22,23 They indicate that CuGaO2

has an indirect band gap and that the near-Fermi level is occupiedby Cu 3d and O 2p. Note that CuCrO2 has a lower synthesistemperature of about 600 1C when using the sol–gel method.Therefore, the CGCOx system can be prepared at a relative lowtemperature via Cr-doping. In addition, the delafossite structurecan be maintained by Cr–Ga substitution for the similar ionicradii of Cr and Ga. Furthermore, in order to further optimize thedelafossite material or device performances, the properties ofthe CGCOx system should be further researched. However, fewstudies about the influence of the chemical composition on theelectronic and optical properties of the CGCOx system have beencarried out. The nature of electronic transitions is still not wellunderstood and needs to be examined carefully and systematicallyby means of first-principles calculations and the correspondingexperiments. By exploring these physical mechanisms, one canmore reasonably regulate the performance of CGCOx-basedoptoelectronic devices.

In this study, first-principles calculations were performed toinvestigate the influence of the chemical composition on theelectronic transitions of the CuGa1�xCrxO2 (0 r x r 1) system.Moreover, the CGCOx films were deposited using a sol–gel methodand the phonon vibration was investigated using infrared

reflectance spectroscopy. Finally, the optical band gap of theCGCOx films was systematically studied using transmissionspectroscopy in the temperature region 10–300 K. This studyshows that the electronic structure and optical properties of thefilms were significantly affected by the covalent interaction ofCr 3d–O 2p–Cu 3d, which can be well conformed by thetheoretical pictures.

2 Theoretical and experimentalsection2.1 Computational details

Delafossite CuMO2 TCOs crystallize in either a hexagonal (spacegroup P63/mmc) or a rhombohedral (space group R%3m) structure.The energy difference between the two structure can be negligible.24

Therefore, hexagonal unit cells of CuGaO2, CuGa0.5Cr0.5O2 andCuCrO2 were modeled to save the computing resources. Theelectronic structure and density of states (DOS) were calculatedby means of the DFT based on the generalized gradientapproximation (GGA) using the Perdew–Burke–Ernzerhof (PBE)functional.25 The integrals over the Brillouin zone (BZ) wereapproximated by using a 10 � 10 � 2 Monkhorst–Pack k-pointmesh. An energy cutoff of 500 eV for the plane-wave expansionof the projector-augmented wave (PAW) was used. Both cellparameters and atomic coordinates were optimized until theresidual forces on each atom were less than 0.01 eV Å�1.

2.2 Fabrication of CGCOx films

CuGa1�xCrxO2 (CGCOx, x = 0, 0.25, 0.5, 0.75 and 1, or in short,CGCO0, CGCO0.25, CGCO0.5, CGCO0.75, CGCO1, respectively)films were directly deposited on (001) sapphire substrates usinga modified sol–gel method. The raw materials gallium nitrate[Ga(NO3)3�5.97H2O, 99.9%], chromium nitrate [Cr(NO3)3�9H2O,99%] and copper acetate [Cu(CH3COO)2�H2O, 99%] were mixedin stoichiometric amounts. Anhydrous ethanol (C2H6O, 99.7%)and ethylene glycol (C2H6O2, 99%) were used as solvents.Ethanolamine (C2H7NO, 99%) was used as a chelating agent.All of the chemical reagents used in this experiment werepurchased from commercial sources and were of analytical gradewithout any further purification. After the CGCOx precursorsolutions became stable and homogeneous by magnetic stirring,they were adjusted to 0.2 mol L�1 and filtered by using a0.22 mm pore size filter. Prior to deposition, the substrates werecleaned with acetone and ethanol in an ultrasonic bath sequen-tially, and then dried in a pure nitrogen stream. The films weredeposited via spin coating onto the sapphire substrates at aspeed of 4000 rpm for 20 s. Each layer of the films was preheatedat 320 1C for 300 s in air to evaporate the organics. The aboveprocess was repeated several times to obtain the desired thick-ness. Next, the films were annealed at 800 1C for 30 min in a N2

flow to crystallize them via a rapid thermal annealing procedure.

2.3 Characterization methods

The crystalline structure of the CGCOx films was analyzed byusing a glancing angle X-ray diffractometer (XRD, D/MAX-2550V,

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Rigaku Co.). The surface morphology and cross-sectional imagesof all the films were examined using atomic force microscopy(AFM: Digital Instruments Icon, Bruker) and scanning electronmicroscopy (SEM: Philips XL30FEG). X-ray photoelectron spectro-scopy (XPS) measurements were carried out on a RBD upgradedPHI-5000C ESCA system (Perkin-Elmer) with Mg-Ka radiation(hn = 1253.6 eV) to investigate the valence states. The infraredreflectance spectra were recorded by using a Bruker Vertex 80 VFTIR spectrometer equipped with a specular reflectance setup. Thespectra were measured in the wavenumber range 50–1000 cm�1

with a resolution of 4 cm�1. The transmittance spectra at 10–300 Kwere measured by using a double beam ultraviolet-infrared spectro-photometer (Perkin Elmer UV/Vis Lambda 950) in the wavenumberrange 190–2650 nm with a spectral resolution of 2 nm. The sampleswere mounted on a cold stage of an optical cryostat (Janis SHI-4-1)and thus cooled to 5 K.

3 Results and discussion3.1 Electronic structure calculations

The calculated band structure and DOS for CuGaO2 and CuCrO2

are shown in Fig. 1. As shown in Fig. 1(a), the conduction-bandminimum (CBM) is at G and the valence-band maximum (VBM)is found at the H point for the CuGaO2 structure, which agreeswell with the previous calculations.24 This indicates that CuGaO2

is an indirect band gap material. It has been found that directG–G, M–M and A–A transitions are forbidden in the delafossitestructure with absorptions only occurring at the L, H and Kpoints. The calculated band gap at the L point is 3.67 eV, whichis similar to the experimental direct optical gap of the CuGaO2

film. As for the DOS, it can be clearly seen that the top of thevalence band and the bottom of the conduction band consist ofmainly O and Cu states with a more Cu character, respectively.The DOS from Cu is dominated by 3d states between �4 and�2 eV indicating a degree of covalent bonding with oxygen. Thestrong hybridization is at the origin of the covalent bonding inCuGaO2. The hybridized Cu d and O p antibonding states of thevalence-band edge are the main reason for the relatively highermobility of holes in Cu(I)-based oxides than in other oxides.26 Forthe Ga element, a long band tail at the bottom of the conductionband and the states around 6 eV above the Fermi level areattributed to the unoccupied Ga s band. The Ga d band isoccupied, which is situated at a much higher binding energybelow the Fermi level. Therefore, the direct electronic transitionis mainly attributed to the Cu and O orbitals.

In Fig. 1(b), the band structure for CuCrO2 is visibly differentto that for CuGaO2. The band structure also exhibits an indirectband gap, whereas the direct gap is found near M in thedirection of G. The qualitative positions of the fundamentaltransition are in good agreement with the consequencereported by David O. Scanlon et al.,12 who also used a hexagonalpolymorph and GGA to simulate the case of CuCrO2. ForCuCrO2, there is an experimentally demonstrated redshift inthe optical absorption relative to the CuGaO2 structure.20 Fromfurther research into the electronic densities of states, the

reason for the extended band structure feature for CuCrO2

becomes apparent. The DOS of CuCrO2 suggests a significantnumber of states at the Fermi level, which are dominated by Crorbitals. The Cr states and the Cu states can be hybridized witheach other via O states in the CuCrO2 structure.21,27 In addition,the Cu states from �2 to �4 eV for CuGaO2 are less pronouncedand present a higher main peak located at �3 eV for CuCrO2.The VBM is dominated by the Cr states, whereas the moredispersed Cu states are pushed to lower energies relative to theVBM, leading to a flat valence band. Similarly, the bottom of theconduction band is predominantly contributed by the Cr statesboth with some Cu and O states, causing the flattening ofthe CBM.

Fig. 2 shows the band structure of CuGa0.5Cr0.5O2 for botha-spin and b-spin, respectively. Obviously, the band structurefor the a-spin channel has a strong resemblance to that forCuCrO2 in Fig. 1(b). However, the band gap for the b-spin isalso nearly the same as that for CuGaO2 in Fig. 1(a). Thisphenomenon can be found in the (Fe/Cr)2O3 system, which canbe ascribed to the strong localization of the Cr states.28 More-over, the calculated band gap is indirect and the value isunchanged from the CuGaO2 structure. However, the CBM forboth spins is located at G, no longer near M for the CuCrO2

structure. The VBM is present just off H along H–A and the VBMfor the a-spin is higher than that for the b-spin. This variationindicates that the valence band level position can be raised byCr-doping, thus decreasing the optical transition energy.

Fig. 1 Calculated electronic band structure and density of states for(a) CuGaO2 and (b) CuCrO2.

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The PDOSs of the CuGa0.5Cr0.5O2 structure for both a-spin andb-spin components are shown in Fig. 3. Based on the introductionof Cr into the Ga lattice sites in alternating ab layers, the oxygenatoms are divided into two categories: O1 and O2. The O1 patternis located between Cu and Ga, while the O2 one makes aconnection with Cu and Cr. Interestingly, the O1 2p state inFig. 3(e) is similar to the DOS of O for the CuGaO2 structure. FromFig. 3(f), a lager shoulder at �0.5 eV for the a-spin component isincreased due to the O2–Cr interaction. Beyond that, the O2 2pstate is spin polarized, which is affected by the formed stablebond with spin polarized Cr ions. It demonstrates that the localO coordination environment is only dependent on the peculiarityof the directly contacted atoms. Moreover, the Cr3+ (3d3) ioniccharge state has a multiple-electron energy-level structure. Theapproximately octahedral crystal field can reduce orbital degen-eracy and split three d electrons into the occupied t2g state and

the empty eg state.29,30 As a consequence, three occupied singleelectron Cr 3d states migrate to the top of the valence band, asshown in Fig. 3(d). The unoccupied a-spin Cr 3d states, eg,constitute the conduction band bottom, whereas the b-spin bandstates, t2g, migrate to the high energy of the conduction band.22,31

This effect is assigned to the ferromagnetic ground state.32

Moreover, copper (Cu) contributes less to the top of thevalence band compared to the CuGaO2 structure. On the contrary,oxygen O2 in particular, occupies more states in the CBM and VBM.The Ga atoms are unable to produce this effect. The difference inchemical bonding between CuGaO2 and CuCrO2 is responsible forthe significant deviation of the band structure and density of states.For CGCOx, the longer Cr–O bonds generate strain on the lattice.A stronger covalent interaction between Cr 3d and O 2p and anindirect Cr 3d–Cu 3d hybridized interaction occur.33 Hence, thepartial charge from O2 shifts to Cr and the charge transfer fromO2 to Cu decreases. Beyond that, the Cr 3d states dominate at thetop of the valence band and the bottom of the conduction band isenhanced significantly with increasing Cr composition. This resultsin a weakening of the Cu 3d + O 2p - Cu 3dz2 + 4s transition.Moreover, the states in the Cu–O–Cr–O–Cu linkages should bemixed effectively by heavy doping, which further moves the valenceband upwards and the conduction band downwards, thus resultingin a lower optical band gap.

3.2 Structure and surface analysis

Fig. 4(a) shows the XRD patterns of the CGCOx films withdifferent Cr concentrations. All samples were measured using aglancing angle configuration to eliminate the contribution ofthe substrate. It can be found that all films are polycrystallinewith a stronger (012) diffraction peak, which locates at about36.41. Furthermore, the intensity of (012) peaks enhanced withthe increase in the Ga constituent. Besides the strong feature,several other weaker diffraction peaks (006), (104), (105), (110)were observed. Note that the variation trend of the (006) peakwith composition x is abnormal, which may be attributed tothe polycrystalline growth. Taking the standard XRD pattern ofCuGaO2 (JCPDS card No. 41-0255) into consideration, all of the

Fig. 2 Calculated electronic band structure of CuGa0.5Cr0.5O2 for (a) a-spinand (b) b-spin.

Fig. 3 The electronic density of states for CuGa0.5Cr0.5O2. (a) Total PDOS,(b) Cu 3d, (c) Ga 4s, (d) Cr 3d, (e) and (f) O 2p.

Fig. 4 (a) XRD patterns of the CGCOx films grown on (001) sapphiresubstrates. The symbol (*) indicates the observed trace of the sapphiresubstrate. (b) The rhombohedral unit cell of the delafossite structure.

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films show a pure rhombohedral phase structure. No impurity phaseis observed in the films except for the CGCO0 film, indicating thatthe Cr atoms have been successfully incorporated into the CGCOxhost lattice. Fig. 4(b) shows the supercell of the CGCOx rhombo-hedral structure. Note that each Cu atom is linearly coordinatedbetween two oxygen atoms, forming O–Cu–O dumbbells along thec axis, which agrees well with the main peak of the XRD patterns.

Fig. 5(a) and (e) show the three-dimensional AFM images ofthe CGCO0.25 and CGCO1 films, respectively. The film grains areclosely gathered and densely arranged, which indicates that all thefilms exhibit nanocrystalline growth. The root-mean-square (RMS)surface roughness is estimated to be 6.39, 4.56, 6.73, 12.8, and16.7 nm for the Cr concentration at x = 0, 0.25, 0.5, 0.75, and 1,respectively. It can be seen that the RMS surface roughnessdecreased first, and increased with increasing Cr-composition.The grain size change shows a similar variation trend, which isattributed to the effect induced by Cr-substitution. It indicates thatthe crystal quality was increased by light doping, whereas the heavydoping resulted in more crystal defects. Scanning electron micro-scopy (SEM) is another common technique for investigating themorphology of the films. Cross-sectional SEM can be applied tomeasure the thickness of the films. The plane-view SEM images ofthe CGCOx films are shown in Fig. 5(b)–(d), (f) and (g), respectively.Note that the grain size is homogeneous and varied with thechange in Cr-composition. The particle gaps decreased first,and increased with increasing Cr-composition. In addition, theCGCO0.25 film has the smallest gaps, which agrees well withthe RMS surface roughness. The remarkable variation in sur-face morphology can affect the optical and electrical properties,such as lattice vibrations and electronic transitions. In thecross-sectional image, an obvious interface between the filmand sapphire substrate can be observed and the thickness ofthe CGCO0.25 film is about 240 � 10 nm.

3.3 Chemical bond and valence states

In order to understand the molecular structure, chemical bondand valence states of the CGCOx films, XPS and infraredreflectance spectra were obtained. The IR spectra of the CGCOx

films recorded at room temperature are shown in Fig. 6. As weknow, the primitive rhombohedral unit cell of delafossitecompounds contains four atoms, giving rise to 12 normalmodes.34,35 Group-theoretical analysis decomposes a generalmode at the Brillouin zone center G as = A1g + Eg + 3A2u + 3Eu.Eu and A2u symmetries are infrared active and include acousticmodes, in which both oxygens must move in the phase.36 InFig. 6(a), the observed infrared active modes near 502 cm�1 and733 cm�1 of the CGCO1 film can be inferred to the Em

u and Am2u

vibrations, respectively. For the CGCO0 film, the Am2u mode is

found clearly at 655 cm�1. The Emu mode around 500 cm�1 is too

weak to be discovered, which agrees well with previousstudies.36 The displacement patterns of the Em

u and Am2u at the

Brillouin zone center are shown in the inset of Fig. 6(a). It is nowonder that the vibration is related to the O–Cu–O–M units.

Interestingly, the intensity of the Emu peaks for the CGCOx

films becomes stronger with increasing x, as shown in Fig. 6(b).However, the peak position is almost unchanged. For the Am

2u

phonon mode the peak position shifts from 655 cm�1 to 733 cm�1

obviously [Fig. 6(c)]. The O–Cu–O bond is seriously affected by thesubstitution of Ga with Cr, which can be ascribed to the lower Crmass and stiffer Cr–O bonds. The double degenerate E modeexpresses movement in the perpendicular direction relative tothe c-axis. However, the A mode describes the vibration in thedirection of the O–Cu–O bonds along the hexagonal c-axis.Therefore, the Em

u mode frequency increment is not as pronouncedas that of the Am

2u mode.37 This indicates that a structural transitionstimulated by Cr-doping would not modify the octahedral environ-ment of the Ga atoms. Nevertheless, the element Cu may reducethe site symmetry for the Cr–O–Cu–O units.

As an example, Fig. 7(a) displays the overall core level XPSsurvey spectra of the CGCO0.5 film. The intense peaks of Cu 2p,Cr 2p, Ga 3d and O 1s can be observed. The C 1s peak at284.6 eV is used as an internal standard to correct the bindingenergy position of other elements. Fig. 7(b–e) show the Lorentzian–Gaussian dividing peak analysis of the Cu 2p, O 1s, Cr 2p and Ga 3dpeaks, respectively. In Fig. 7(b), the 1/2 and 3/2 spin–orbit doubletcomponents of the Cu 2p photoelectron are found to be located at

Fig. 5 Three-dimensional AFM images of (a) CGCO0.25 and (e) CGCO1 films. Note that the different scale heights are given in the pictures and themeasured area is 1 � 1 mm2. The plane-view SEM images of (b) CGCO0, (c) CGCO0.25, (d) CGCO0.5, (f) CGCO0.75 and (g) CGCO1 films. The picture (h)shows the cross-sectional image of the CGCO0.25 film. Note that the same scale is given in the picture.

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about 952.4 eV and 932.2 eV for the CGCO0.5 film, respectively.34

Note that there is no shake-up line of Cu 2p3/2 in the spectra. Thisindicates that the valence of the Cu ions is +1 in the present CGCOxfilms. The O 1s peak is deconvoluted into two individual peaks[Fig. 7(c)]. The lower binding energy peak located at about 530 eV isattributed to the lattice oxygen, whereas the higher binding energypeak positioned at about 532 eV should be attributed to the surfaceadsorbed hydroxyl oxygen. In Fig. 7(d), the Cr 2p peaks at bindingenergies of 576.9 and 586.8 eV for the CGCO1 film can be assignedto Cr 2p3/2 and Cr 2p1/2 peaks, respectively. The Ga 3d peaks atbinding energies of 20.6 eV (Ga 3d5/2) and 22.7 eV (Ga 3d3/2) for the

CGCO0 film can be found in Fig. 7(e). The XPS data suggest thatboth Cr and Ga ions possess +3 valence states.

It is well known that the binding energy is closely related tothe chemical environment of the atoms. The combination ofdifferent element types and quantities will change the bindingenergy of inner shell electrons. It can be seen that the peakpositions of Cu 2p and O 1s show a blueshift, whereas the peakpositions of Cr 2p and Ga 3d show a redshift with increasing Crcomposition. This means that the microstructure of the CGCOxfilm can be changed by Cr-substitution, which is attributed tothe hybridization of Cr 3d states with O 2p energy levels. Thisindicates that an indirect Cr–Cu interaction is modified by theCu–O–Cr bond, which agrees well with the above theoreticalcalculations. Moreover, the Cu–O and Ga–O bonds for CGCOxstructures can be affected by the shorter Cr–O bond. The Ga–Odistance becomes longer, whereas the Cu–O (Cr) lengthdecreases. Therefore, the binding energy of Cu 2p and O 1s isenhanced, while the Ga 3d related core levels are decreased byCr-doping.

3.4 Optical transitions

Fig. 8(a) shows the transmittance spectra for the CGCOx filmsat room temperature. It can be clearly seen that each film has arelatively high optical transmittance in the visible range. Notethat the CGCO1 film has a mild absorption edge, whereas theCGCO0 film has the steepest absorption edge, which can beattributed to the introduction of the Cr element. The absorp-tion edge for the CGCOx films shows a redshift trend withincreasing Cr composition, which is assigned to the hybridiza-tion of Cr 3d states and O 2p states from the first-principlescalculations. However, an abnormal absorption edge appearsfor the CGCO0.25 film. This indicates that there are twoelectronic transitions in this region. The transmittance spectraof the CGCO0.25 film at different temperatures are plotted inFig. 8(b). The enlarged transmittance spectra from 10 K to300 K for the CGCO0.25 and CGCO0.5 films near the absorptionedge are shown in Fig. 8(c) and (d), respectively. Anomalously,the absorption edge for the CGCO0.25 film shows a blueshifttrend with increasing temperature at low temperatures and thelargest absorption edge appears at about 125 K. However, theabsorption edge for the CGCO0.5 film shows a redshift trendwith increasing temperature, which is also found in other films(not shown).

In order to understand the temperature dependence of theband-gap behavior in detail, the optical band gap energies wereinvestigated. The optical band gap, named EOBG, is deduced byTauc’s relationship (ahn)n = A(hn � EOBG). The EOBG can beestimated by an intercept of the (ahn)n plot.38 a denotes theabsorption coefficient, which can be obtained using the rela-tionship: a = �ln(T)/d. T, d and A are the transmittance of thefilms, the thickness and a constant, respectively. The exponentn determines the type of optical transitions, with n = 2 for adirect band gap (Edir

OBG) and 1/2 for an indirect band gap (EindirOBG )

transition, respectively.For the CGCO0.5 film, the Edir

OBG can be estimated to be3.20 eV at 10 K and 3.13 eV at 300 K, while the Eindir

OBG is 2.86 eV

Fig. 6 (a) Infrared reflectance spectra of the CGCOx films at room tem-perature. The dashed lines show the vibration trend of the phonon modes.(b) An enlarged reflectance spectra region between 485 and 545 cm�1.(c) IR spectra of the CGCO films in the range of 636–800 cm�1.

Fig. 7 (a) Overall core level XPS spectra of the CGCO0.5 film. XPS spectraof the Cu 2p (b), O 1s (c), Cr 2p (d) and Ga 3d (e) regions for the CCGOxfilms. Note that the dotted lines represent the nonlinear fitting results.

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and 2.79 eV, respectively [Fig. 9(a) and (b)]. In Fig. 1, it can beseen that both Cu 3d and O 2p states contribute to the upperpart of the valence band and the bottom of the conductionband for CuGaO2. In particular, the CBM is mainly dominatedby the Cu 3dz2 and 4s characters.12,24 However, it can be foundthat the near-Fermi-level structure is primarily dominated by Cr3d states in the structure which has both Cu and Cr elements,as shown in Fig. 3. Thus, the direct electronic transition of theCGCO0.5 film can be ascribed to the excitation of the 3d - 3dtransition, which is attributed to the covalent interactionbetween Cr 3d and O 2p orbitals.22 It is worth noting that theoptical band gap shows a redshift with increasing temperaturefor both direct and indirect electronic transitions, which isattributed to the lattice thermal expansion and the electron–phonon interaction.39 The distance between the lattices gradu-ally increases with temperature and the lattice wave frequencychanges with the crystal volume.40,41 Moreover, the valenceband bottom represents an upward movement as the electronsof the semiconductor are energized, resulting in the modifica-tion of the electronic band structure.

Next we turn our attention to the abnormal absorption edgeof the CGCO0.25 film. The characteristic is only observed for0 o x r 0.3 and the absorption becomes weaker with theincreasing composition of Cr. To investigate the origin of thisextraordinary phenomenon, the temperature dependence of theEdir

OBG for the CGCO0.25 film has been plotted in Fig. 10(a). It isworth noting that there are two interband transitions, named Eg1

and Eg2, respectively. The Eg1 transition corresponds to thestrong absorption and can be estimated to be 3.31 eV at 300 K,which is slightly higher than the Edir

OBG of the CGCO0.5 film. Asshown in Fig. 8(a), the strong absorption for CGCOx films can bemodified by Cr doping, which indicates that the strong absorp-tion corresponds to the essential optical feature. Therefore, theEg1 energy band is attributed to the 3d - 3d transition from the

valence-band maximum (VBM) to the conduction-band mini-mum (CBM) after introducing the Cr element [Fig. 10(c)].42,43

In Fig. 10(d), the Eg1 band gap shows a redshift trend withincreasing temperature. However, it can be observed that theband gap energy decreases with temperature in the low tem-perature region, whereas the variation is much less than thatfrom 150 K to 300 K from Fig. 10(e), which can be attributed tothe lattice thermal expansion. With increasing temperature, theinteratomic distance along the direction of the propagation can bechanged by the longitudinal phonons and the lattice thermalexpansion becomes stronger.40 Note that these mechanisms become

Fig. 8 (a) Transmittance spectra of the CGCOx films at room tempera-ture. The yellow circle shows an abnormal absorption. (b) Transmittancespectra of the CGCO0.25 film at different temperatures. An enlargedspectral region near the absorption edge for CGCO0.25 (c), and CGCO0.5(d) at different temperatures.

Fig. 9 (a) Plots of (ahn)2 vs. the photon energy for the estimation of directoptical band gap energies from the CGCO0.5 film. The inset shows theelectronic transition diagram. (b) Plots of (ahn)1/2 vs. the photon energy forthe estimation of indirect band gap energies from the CGCO0.5 film. (c) Anenlarged region for the indirect band gap.

Fig. 10 (a) Plots of (ahn)2 vs. the photon energy for the determinations ofdirect optical band gap energies from the CGCO0.25 film. (b) Temperaturedependence of the Eg2 energy band. (c) Electronic transitions diagram for theCGCO0.25 film. (d) An enlarged region for the Eg1 transition. (e) Temperaturedependence of the Eg1 energy band.

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190 | J. Mater. Chem. C, 2017, 5, 183--191 This journal is©The Royal Society of Chemistry 2017

increasingly important with the temperature, in order to increase thevariation rate of the band gap.

The Eg2 transition can be ascribed to the absorption edgeregion. At room temperature, the energy band Eg2 at about3.61 eV can be found, which agrees well with the band gap ofCGCO0 in the previous reports.44 Note that the band structureof delafossites will be modified by M-site alloying, while theessential optical features remain unchanged for light doping.The retention of the direct band gap for CGCO0 after alloyingcan be expected based on similar findings in the predictedband structure CuGa1�xFexO2.45 Therefore, the Eg2 transitionmay be ascribed to the dipole-allowed Cu 3d + O 2p - Cu 3dz2 + 4stransition [Fig. 10(c)],22,46 which corresponds to the L–L transitionin Fig. 1(a).

More remarkably, the Eg2 energy band shows a non-monotonictrend with increasing temperature [Fig. 10(b)]. The minimumband-gap energy appears at the highest temperature of 300 K,whereas the maximum energy appears at about 125 K, insteadof the lowest temperature of 10 K. This anomalous temperaturebehavior was also observed in other materials with monovalentCu or Ag cations, such as Cu(Cr/Mg)O2 and AgGaSe2.41,47 Allthese materials possess a maximum gap energy at a relativelylow temperature. Note that the anomalous temperature varia-tion in the low temperature region is related to p–d electronhybridization.48 The p–d hybridization will be further enhancedby the occupied Cr 3d states interacting covalently with theneighboring O 2p states, which makes the abnormal pheno-menon much clearer.49 Extraordinarily, the Eg2 energy band canbecome weaker by increasing the Cr doping composition,which is attributed to the stronger interaction Cr 3d–O 2p–Cu3d.41 Measurements at low temperatures using crystals withpairwise replacement of isotopes would help to separate theindividual effects of the various atoms on the zero-temperaturegap renormalization.

4 Conclusion

In summary, the electronic structure of the CGCOx system wasinvestigated theoretically using first-principles calculations andtemperature-dependent transmittance spectroscopy for the firsttime. It can be found that the relevant electronic transitionscalculated using the exchange–correlation potential of the GGAtype in the PBE version agree well with the experimental datafor the CGCOx films. Replacing Ga with Cr results in theappearance of Cr 3d states. The Cr–O interaction indirectlychanges the Cu–O bonds, which causes an increase in thedensity of 3d states at the top of the valence band and thebottom of the conduction band. In addition, the vibrationfrequency of infrared active phonon modes and the bindingenergy of Cu 2p and O 1s increase with Cr composition due tothe lower Cr mass and stiffer Cr–O bonds. Moreover, theabnormal absorption in CGCO0.25 is attributed to the Cu3d + O 2p - Cu 3dz2 + 4s transition. The non-monotonic trendof the Eg2 transition in the low temperature region is attributedto the covalent interaction Cr 3d–O 2p–Cu 3d and should

become weaker with more Cr-doping. Remarkably, the spin–orbit interactions of the Cr3+ ions in an octahedral environmentincrease the density of states at the CBM and VBM for CGCOx(0 o x r 1). Therefore, the optical absorption edge is dominatedby the excitation of the 3d - 3d transition. This can bring outthe overlap of 3d states, thus increasing the mobility of p-typecharge carriers.

Acknowledgements

This work was financially supported by the Major State BasicResearch Development Program of China (Grant No. 2013-CB922300), the Natural and Science Foundation of China(Grant No. 11374097, 61376129 and 61504156), the Projects ofScience and Technology Commission of Shanghai Municipality(Grant No. 15JC1401600 and 14XD1401500), and the Programfor Professor of Special Appointment (Eastern Scholar) at ShanghaiInstitutions of Higher Learning.

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