Article pubs.acs.org/JPCC
Lithium Ion Conducting Boron-Oxynitride Amorphous Thin Films: Synthesis and Molecular Structure by Infrared Spectroscopy and Density Functional Theory Modeling M. Dussauze,*,†,‡ E. I. Kamitsos,*,† P. Johansson,§ A. Matic,§ C. P. E. Varsamis,† D. Cavagnat,‡ P. Vinatier,∥ and Y. Hamon∥ †
Theoretical and Physical Chemistry Institute, National Hellenic Research Foundation, 48 Vassileos Constantinou Avenue, 116 35 Athens, Greece ‡ Institut des Sciences Moléculaires - UMR 5255 CNRS, Université Bordeaux I, 351 Cours de la Libération, 33405 Talence Cedex, France § Department of Applied Physics, Chalmers University of Technology, SE-412 96, Göteborg, Sweden ∥ CNRS, Université de Bordeaux, ICMCB site de l’ENSCBP-IPB, 87 Avenue du Dr. A. Schweitzer, Pessac, F-33608, France S Supporting Information *
ABSTRACT: Li ion containing oxynitride amorphous thin films are promising materials for electrochemical applications due to their high ionic conductivity, mechanical stability and chemical durability. Here we report on the preparation of Li boron-oxynitride (LiBON) amorphous thin films by rf sputtering of Li-diborate and Li-pyroborate targets in nitrogen atmosphere. The materials produced were subsequently studied by infrared transmittance spectroscopy assisted by density functional theory calculations using representative Li boron-oxide and boron-oxynitride clusters. The combination of experiments and calculations allows us to propose accurate vibrational assignments and to clarify the complex infrared activity of the LiBON films. Both experimental and calculated spectra show that nitrogen incorporation induces significant structural rearrangements, manifested mainly by a change in boron coordination number from four to three, and by the formation of boron−nitrogen-boron bridges. The nature of boron−nitrogen bonding depends on the composition of the sputtering target, with an exponential relationship adequately describing the dependence of B−N stretching frequency on bond length. Besides bonding to two boron atoms by covalent bonds, the nitrogen atoms interact also with Li ions by participating in their coordination sphere together with oxygen atoms. Likely, boron−nitrogen bonding in LiBON films facilitates Li ion transport due to induced charge delocalization within the boron−nitrogen-boron bridges and reduced electrostatic interaction with the Li ions. A thickness-dependent ionic conductivity was reported7 for Li-borate films developed by ion-beam sputtering from a 0.2Li2O-0.8B2O3 glass target. The direct current (dc) ionic conductivity increased by about 3 orders of magnitude upon reducing the film thickness from 120 to 7 nm. Possible mechanisms proposed to explain this effect included structural modifications at the interfaces, formation of space-charge regions at the interfaces and establishment of randomly distributed ion-conducting channels within the glassy film. Doping Li-borate glasses with lithium salts also improves the ionic conductivity.1,2 Amorphous films of composition xLi2SO4(1−x)LiBO2 with x = 0.4 to 0.8 were developed by radio frequency (rf) magnetron sputtering, and found to exhibit
1. INTRODUCTION Thin films of ionic conducting oxide glasses constitute potential candidates for solid-state electrochemical applications including batteries, sensors, and electrochromic displays.1−5 Since ionic conductivity is a key physical property for these applications, the field early focused on improving ionic conductivity by techniques that allow for the development of films with large contents of mobile metal ions. This is, for example, the case of xLi2O-(1−x)B2O3 thin films developed by thermal evaporation with the Li2O mole fraction spanning the range 0.52 ≤ x ≤ 0.85.6 The ionic conductivity of ∼0.3 μm thick films was found to increase exponentially with lithium ion content and to reach values as high as 1 × 10−7 Ω−1 cm−1 at room temperature for x = 0.72. This enhancement of ionic conductivity was attributed to the increasing density and mobility of Li ions as the threedimensional borate network is gradually disrupted at high Li2O contents. © 2013 American Chemical Society
Received: February 12, 2013 Revised: March 16, 2013 Published: March 21, 2013 7202
dx.doi.org/10.1021/jp401527x | J. Phys. Chem. C 2013, 117, 7202−7213
The Journal of Physical Chemistry C
Article
room temperature dc conductivity as high as 2.5 × 10−6 Ω−1 cm−1 for x = 0.7.8 Furthermore, rf-sputtering can produce stable amorphous electrolytes in greatly extended glass-forming regions relative to those established for conventional meltquenched glasses.9,10 In addition, a gradual substitution of oxygen by nitrogen atoms in the structure of Li phosphate glasses was found to significantly increase the ionic conductivity,11,12 besides improving the mechanical properties and chemical durability.13−19 Bates and co-workers11 were the first to report the formation of an amorphous thin film material with composition Li3.3PO3.9N0.17 by sputtering a Li3PO4 target in pure N2 atmosphere. This is the so-called LiPON electrolyte with a Li ion conductivity of 2 × 10−6 Ω−1 cm−1 at 25 °C and stable at potentials exceeding 5 V versus metallic Li.11 Partly due to the promising characteristics of LiPON, the field of nitrogencontaining thin film electrolytes continues to attract intense interest.19 Compared to LIPON thin films there are only a few reports on the borate analogues, lithium boron-oxynitrides (LiBON).20−22 Birke et al.20,21 showed that a LiBO2 target can be nitrided by sputtering in N2 atmosphere to give a thin film electrolyte with an estimated composition of LiBO1.86N0.09. A more recent study showed that the amount of nitrogen in LiBON thin films can increase substantially by means of rf sputtering at constant nitrogen pressure while increasing the nitrogen flow rate.22 Using LiBO2 targets up to one-third of the initial oxygen content can be replaced by nitrogen atoms and thereby produce LiBO1.3N0.6 films with ionic conductivities increasing with the nitrogen content, reaching 7.5 × 10−8 Ω−1 cm−1 at 25 °C (for an intermediate N2 flow rate of 20 mL/ min). Although the overall family of oxynitride glasses indeed has been the subject of several investigations during the last three decades,11−28 open questions still remain with respect to their structure and, in particular, the details of nitrogen bonding within the glassy network, as well as the origin of the ionic conductivity enhancement. Here we present a molecular level structural investigation of LiBON thin films by infrared (IR) spectroscopy, a technique proven to be very effective in understanding formation and structure in Li-borate amorphous thin films and other glassy electrolytes,29,30 and due to the structural complexity of oxynitride materials16 further supported by density functional theory (DFT) calculations31 using representative Li-borate and Li-boron-oxynitride clusters.
process, a potential energy distribution (PED) analysis was performed to also quantify the contribution of internal coordinates to a specific vibrational mode. The DFT method employed was B3LYP/6-311+G*, which gives IR spectra of high quality and with a frequency scaling factor of 0.99 or better; hence no scaling was applied here. Artificial broadening has been applied to the computed IR spectra only in order to assist visual interpretation. The Gaussian03 software package was used for all computations.32
3. EXPERIMENTAL SECTION 3.1. Materials Synthesis. Thin films were prepared by magnetron rf reactive sputtering under nitrogen atmosphere using two different Li-borate xLi2O-(1−x)B2O3 targets; lithium diborate (x = 0.33 or Li2B4O7) and lithium pyroborate (x = 0.67 or Li4B2O5). As demonstrated first by Bates et al.,11 rf reactive sputtering is an effective method to form oxynitride thin films. The lithium diborate target was prepared from reagent-grade powders of crystalline B2O3 and LiBO2, while LiBO2 and Li3BO3 were used for the lithium pyroborate target. Stoichiometric amounts of the starting materials were mixed thoroughly in an agate mortar and pressed uniaxially at 900 kg/ cm2 to form pellets 5 cm in diameter. Each pellet was subsequently annealed twice at 800 °C for 12 h with additional grinding and pressing between the two annealing processes. This procedure results in high density targets, which is a key experimental parameter to reach high deposition rates.25 Films were deposited under 1 Pa of nitrogen with a target-tosubstrate distance of 6 cm and with rf power ranging from 1.5 to 2.5 W/cm2. The nitrogen flow rate was set at 45 mL/min, which was the highest attainable rate for the pumping system. As noted above, nitrogen incorporation increases with flow rate even when the nitrogen pressure is kept constant.22 Films were sputtered on silicon wafers having both surfaces optically polished and with (100) orientation and 0.6 mm thickness. The duration of deposition was controlled to produce films with thicknesses in the range 1.0 to 1.8 μm which is suitable for IR transmittance measurements.29 In order to assist the structural studies of the thin films, also bulk Li-borate glasses xLi2O-(1−x)B2O3 were prepared from stoichiometric amounts of reagent-grade powders of Li2CO3 and B2O3 and with x values ranging from 0.33 to 0.7. The thoroughly mixed batches were melted in Pt crucibles in an electric furnace at temperatures 1000−1200 °C for 30 min. The bubble-free melts were subsequently quenched in a stainless steel mold. After annealing at Tg-50 °C33 for one hour, the glass disks were polished using a Struers polishing machine (LaboPol-1), equipped with the appropriate set of abrasive papers and diamond suspensions which result in surface roughness better than ca. 0.25 μm. 3.2. Measurements and Simulations of Infrared Spectra of Thin Films. All measurements were performed on a Fourier-transform IR vacuum spectrometer (Bruker 113v) appropriately equipped with sources, detectors, and beam splitters to allow for a continuous spectral coverage in the range 30 to 5 000 cm−1. All spectra were measured at room temperature and represent the average of 400 scans. Transmittance spectra of thin films were measured with low resolution (10 cm−1) to average over the interference fringes due to silicon substrate.29 Specular reflectance spectra of bulk glasses were measured with 2 cm−1 resolution at a quasi-normal incidence (11°) against a highly reflective aluminum mirror.
2. COMPUTATIONAL METHODS We performed geometry optimizations of a range of clusters selected to represent the various experimental lithium borate compositions. Subsequently, the Hessians, via the second derivatives of the energy, were calculated for the so obtained stable structures in order to obtain IR spectra. Nitrogen atoms were then incorporated in the clusters while preserving charge neutrality, to account for oxynitride formation. Prior to any geometry optimizations, the dangling bonds originating from singly bonded oxygen atoms were neutralized by adding hydrogen-like atoms with masses equal to boron (HB) in order to provide better agreement between calculated and experimental IR spectra. Qualitative vibrational assignments were made by visualizing the atom displacement vectors for each mode likely responsible for experimental IR bands with emphasis on identifying changes upon nitrogen incorporation. In addition to this 7203
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To understand the structural characteristics of thin films relative to those of bulk glasses having the target composition, comparisons were made between measured and simulated IR spectra of thin films. The simulated spectra were obtained using as input the infrared response functions (n, k) of bulk glasses and Si substrate, where n and k are the real and imaginary parts of the complex refractive index. To this aim, the measured specular reflectance spectra of bulk glasses were analyzed by Kramers−Kronig transformation to obtain the complex refractive index.30,34 For silicon, n and k were determined by the inversion of transmittance and reflectance spectra measured with low resolution.35 The model employed here to simulate the IR spectra of thin films is based on rigorous expressions for transmittance and reflectance of the film/substrate bilayer system and takes into account all optical interference effects occurring in this system. A detailed description of the model is reported in ref 29.
4. RESULTS 4.1. Infrared Spectra of Lithium Boron Oxynitride Thin Films. The measured IR spectra of LiBON thin films are here presented in comparison with spectra simulated using the complex refractive index of Si wafer and bulk Li-borate glasses having the same or similar Li/B ratio with the target material. Using this approach, the thickness of the LiBON film is the only fitting parameter. The model accounts fully for interference fringes, which are superimposed in the midinfrared region on the borate vibrational bands active below ca. 1500 cm−1. Since our previous study on sputtered lithium borate thin films showed lithium deficiency with respect to target compositions,29 a satisfactory agreement needs to be obtained between measured IR spectra of oxynitride films and those simulated for glass compositions with similar Li/B ratios. Then, the fitting procedure can allow for the simultaneous evaluation of different factors including purely optical effects (i.e., interference), a possible lithium deficiency, and structural differences induced by nitrogen incorporation in the borate matrix. Therefore, a close agreement between simulated and experimental spectra of films can yield valuable information on (i) interference fringes above ca. 1700 cm−1 due to the finite thickness of the amorphous films,29 (ii) the nature of borate structural entities through their characteristic vibrational modes in the ca. 500−1500 cm−1 range,34 and (iii) the sites hosting lithium ions by probing the far-infrared active Li ion-site vibrations below ca. 500 cm−1.36 The experimental IR transmittance spectrum of LiBON film formed from the Li-diborate target (0.33Li2O-0.66B2O3) is shown in Figure 1a and is compared with the simulated spectrum for a 1.00 μm thick Li-diborate glassy film having the optical responses n and k of bulk Li-diborate glass. The fact that the measured interference fringes are perfectly reproduced above ca. 1500 cm−1 suggests that all optical effects are taken properly into account.29 Therefore, spectral differences below 1500 cm−1 should correspond to real structural differences between LiBON film (experimental) and Li-borate film (simulated). To facilitate comparison, the spectral profiles are shown in Figure 1b in an enlarged scale below 1700 cm−1. The close agreement in the far-infrared between the simulated spectrum from bulk Li-diborate glass and the measured spectrum for LiBON film indicates that no significant lithium losses occurred during sputtering the Li-diborate target in N2, as opposed to sputtering in argon where lithium losses were observed to be ca. 17%.29
Figure 1. (a) Comparison of the experimental transmittance spectrum of a lithium boron-oxyntride film (red line) with the simulated spectrum of a 1.00 μm thick Li-diborate glassy film (blue line) and (b) the same spectra shown in an expanded frequency scale below 1700 cm−1. The oxynitride film was deposited on a 0.6 mm thick silicon substrate by sputtering a Li-diborate target under N2. The simulated spectrum was obtained from the complex refractive indices of bulk Lidiborate glass and silicon substrate by modeling the film/substrate bilayer system as reported in ref 29a.
The previous study of Li-borate films corresponding to the diborate composition showed that the borate backbone consists of borate triangular and tetrahedral units with strong IR-active asymmetric stretching modes at ca. 1200−1600 and 800−1200 cm−1, respectively, and deformation modes at ca. 600−850 cm−1.29 The sharp band at ca. 610 cm−1 is not related to borate species; it corresponds to the strongest lattice mode of the silicon substrate.37 As observed in Figure 1, there are large differences between experimental and simulated spectra. In particular, the 800−1200 cm−1 profile associated with tetrahedral borate units is drastically reduced in intensity upon nitridation. This is accompanied by an increase of relative intensity in the 1200−1500 cm−1 profile of triangular borate units, which develop also relative intensity at ca. 1500 cm−1 (shoulder). Figure 2 shows the transmittance spectrum of the LiBON film deposited by sputtering in nitrogen a lithium pyroborate target (0.67Li2O-0.33B2O3). It was found that the closest agreement between experimental and simulated spectra is achieved when the complex refractive index of the 0.6Li2O0.4B2O3 bulk glass is used for simulation instead of that for the pyroborate composition. This suggests a ca. 10% lithium deficiency in the oxynitride film with respect to the target material. Bands above 1100 cm−1 for glass 0.6Li2O-0.4B2O3 can be assigned to boron−oxygen stretching modes of the pyroborate (B2O54‑) units.34,38,39 In particular, the high frequency envelop 1330−1600 cm−1 is due to stretching vibrations of B-NBO bonds (NBO = nonbridging oxygen atom), and the lower frequency region 1030−1330 cm−1 to stretching vibrations of the B−O−B bridging bonds of B2O54‑ 7204
dx.doi.org/10.1021/jp401527x | J. Phys. Chem. C 2013, 117, 7202−7213
The Journal of Physical Chemistry C
Article
investigation we will consider next the results of computational modeling. 4.2. Computational Study of Lithium Diborate Clusters. We applied DFT calculations to trace the chemical bonding changes upon nitridation during sputtering and to identify their spectroscopic manifestation. First, for a fixed amount of lithium ions, every two nitrogen ions (N3−) bonded to the borate matrix will remove three oxygen ions (O2−) from the structure to maintain charge neutrality.14 This suggests that bridging oxygen atoms (noted as O1/2) and NBO (noted as O) are replaced by nitrogen according to bonding requirements, that is, by single (−N
