Highly conductive PdCoO2 ultrathin films for transparent electrodes

We report on the successful synthesis of highly conductive PdCoO2 ultrathin films on Al2O3 (0001) by pulsed laser deposition. The thin films grow along the c-axis of the layered delafossite structure of PdCoO2, corresponding to the alternating stacking of conductive Pd layers and CoO2 octahedra. The thickness-dependent transport measurement reveals that each Pd layer has a homogeneous sheet conductance as high as 5.5 mS in the samples thicker than the critical thickness of 2.1 nm. Even at the critical thickness, high conductivity exceeding 104 Scm-1 is achieved. Optical transmittance spectra exhibit high optical transparency of PdCoO2 thin films particularly in the near-infrared region. The concomitant high values of electrical conductivity and optical transmittance make PdCoO2 ultrathin films as promising transparent electrodes for triangular-lattice-based materials.

corundum Al2O3, the close similarities in the triangular lattice motif of surface oxygens enable us to synthesize highly crystalline PdCoO2 thin films.
We have prepared PdCoO2 thin films by pulsed-laser deposition (PLD). Commercially available Al2O3 (0001) substrates (SHINKOSHA CO., LTD) were ultrasonically cleaned by acetone and ethanol, followed by annealing in air at 900 °C for 12 hours to obtain an atomically flat surface with step-and-terrace structures. The annealed substrate was loaded into a vacuum chamber and preheated at T = 700 °C for 10 min under an oxygen partial pressure of PO2 = 100 mTorr. A stoichiometric PdCoO2 target (KOJUNDO CHEMICAL LABORATORY Co., LTD) and a Pd-PdO mixed-phase target, prepared by sintering pelletized PdO powder at 1000 °C for 24 h in air, were ablated alternately by KrF excimer laser with the laser fluence of 2 J/cm 2 under the growth condition of T = 700 °C and PO2 = 100 mTorr. We repeated an ablation sequence of 140 pulses at the laser repetition rate of 5 Hz for PdCoO2 and 300 pulses at 15 Hz for Pd-PdO targets. This single cycle resulted in 0.2 nm deposition on average. We repeated the cycle to fabricate the thin films with desired thickness. The samples were cooled down to room temperature in roughly 10 min immediately after the growth. The cation composition in the films was evaluated by inductively coupled plasma atomic emission spectroscopy (ICP-AES) and energy-dispersive X-ray spectroscopy (EDX) using the calibration line determined by the ICP-AES of a thick (d ~ 200 nm) sample.
We note that thin films prepared with only a PdCoO2 target suffer from significant Pd deficiencies with typical composition ratios of Pd/Co ~ 0.6. Employing the alternate deposition with the Pd-PdO target was found to effectively improve the stoichiometry as Pd/Co = 0.9 ± 0.1. The surface morphology was measured by atomic force microscopy (AFM). Crystal structures of the PdCoO2 thin films were characterized by X-ray diffraction (XRD). The thickness d of the samples was determined by the thickness fringes observed around the PdCoO2 (0006) diffraction peaks. High-resolution transmission electron microscope (HRTEM) images were collected with a JEOL EM-002B. For electrical transport measurement, indium electrodes were mechanically soldered on the samples. The temperature dependence 5 of the sample resistance was measured by a four-terminal method using a Quantum Design, physical property measurement system (PPMS). The transmittance spectra were measured with a Shimadzu UV-3600 plus equipped with an integrating sphere MPC-3100.
The XRD patterns for the PdCoO2 thin films with different thicknesses are shown in Figure 2(a). All the film peaks are assigned to PdCoO2 (0003n), indicating that c-axis oriented PdCoO2 thin films are grown without any traces of impurity phases. The widths of the PdCoO2 (0003n) peaks in the 2- scans become larger with decreasing the PdCoO2 thickness, as expected from the Laue function. In addition, clear interference thickness fringes appear around the main diffraction peaks for all the samples. We determined the film thickness denoted as the numbers in Fig. 2(a) using the periodicity of the fringes near PdCoO2 (0006) peaks. To identify the epitaxial relationship of PdCoO2 and Al2O3, we measured the XRD -scans around the PdCoO2 (011 ̅ 2) and Al2O3 (011 ̅ 2) diffraction peaks. As shown in Fig. 2(b), the -scan diffraction patterns have a 6-fold symmetry for PdCoO2 and a 3-fold symmetry for Al2O3. The peak positions in the -scans of the PdCoO2 (011 ̅ 2) are shifted by  = 30° from that of Al2O3 (011 ̅ 2), indicating the epitaxial relationship with the same oxygen triangular configuration in Fig. 1(d), where the lattice unit of PdCoO2 and Al2O3 are 30-degree rotated from each other. The PdCoO2 thin films showed the 6-fold scan diffraction pattern, despite the 3-fold symmetric crystal structure of delafossite PdCoO2. It is thus reasonable to consider that the PdCoO2 thin film has two-domains with the in-plane orientation different by  = 180°. This is supported by the AFM image of the PdCoO2 surface shown in Fig. 2(c), which detects triangular shapes aligned to two directions with one of the base parallel to the [11 ̅ 00]Al2O3 direction.
These triangular domains are consistent with the two kinds of crystalline orientations found in the XRD -scan and domains A and B in Fig. 1(d).
The lattice periodicity in the PdCoO2 thin film was resolved by HRTEM. As shown in Fig. 2 direction, corresponding to the layered crystal structure of the PdCoO2 thin film. The period of the bright 6 lines is about 0.59 nm, which agrees well with the 1/3 unit cell height, i.e., a single stack of Pd layer and CoO2 octahedron. This periodicity is clearly observed in the diffraction pattern of PdCoO2/Al2O3 shown in the inset of Fig. 2(d). Clear PdCoO2 (0003n) bright spots, indicated by white arrows, are observed together with those for the Al2O3 substrate indicated by gray arrows. We note that the Al2O3 substrate showed normally forbidden (0003) and (0003 ̅ ) spots, which might be due to strained crystal structure with off-stoichiometric oxygen composition 19 caused by annealing in growth process. The periodic HRTEM image evidences that the atomic layers in PdCoO2 are regularly ordered along the PdCoO2 [0001] direction.
The thickness dependence of the room-temperature sheet conductance (1/Rs 300K ) is plotted in Fig. 3(a).
The 1/Rs 300K linearly increases above the thickness larger than d = 2 nm, indicating that each PdCoO2 layer possesses the homogeneous sheet conductance. The room-temperature sheet conductance per Pd sheet, deduced from the slope of the fitting line in Fig. 3(a), is as high as 5.5 mS, which is comparable to that of the doped graphene (~ 8 mS). 20 The existence of a roughly 1-unit-cell dead layer implies that the initial growth unit seems a whole unit cell of PdCoO2 composed of three sequences of the Pd sheet/CoO2 layers, rather than the 1/3 unit cell, i.e., a single sequence of the Pd sheet / CoO2 stack. Formation of such a whole single unit cell might be a key requirement to stabilizing the structure and producing high sheet conductance. After the formation of the 1-unit-cell initial layer, growth unit seems to become 1/3 unit cell as evidenced by steps with the 1/3-unit-cell height in the cross-sectional height profile in Fig. 2(c). We note that a 1-unit-cell-thick dead layer should exist also in the thicker films (d > 2 nm), as indicated by the linear 1/Rs vs d dependence shifted to the positive direction in d axis. Extrinsic scattering origins such as surface roughness and interfacial effects could be responsible for the persisting dead layer.
The temperature dependence of the sheet resistance (Rs-T curve) for d = 2.1 -8.8 nm is displayed in Fig. 3(b). All the samples show monotonic decrease of Rs upon cooling down to T ~ 40 K with small upturns at low temperature. In comparison with polycrystalline and single-crystalline bulk studies, residual resistance ratio (RRR) of about 2 in our films is closer to that in polycrystalline bulk of about 4, 21 7 rather than high purity single-crystal of 400. 14 The plausible origins for such upturns are either impurity in the thin films or carrier scattering at the domain boundaries. Further improvement of crystallinity in PdCoO2 thin films will lead to increase 1/Rs 300K per Pd sheet toward the bulk corresponding value of ~23 mS, 1 by reducing the effect of impurities and the domain boundaries. 14 The optical transmittance spectra of the PdCoO2/Al2O3 in Fig. 4 represent the specific features of the band structure and high optical transparency. The overall spectral shape is identical for all the thicknesses, representing high transparency at near-infrared region at around 1 eV. The transmittance spectra have three characteristic dips around 1.