A polycrystalline platinum pernitride (PtN2) thin-film was successfully synthesized via nitridation of a platinum thin-film deposited on α-Al2O3 substrate at the pressure of ∼50 GPa by using the laser-heated diamond anvil cell. The current–voltage characteristic and optical reflectance of the synthesized PtN2 thin-film were measured under ambient conditions. Combined with first-principles calculations, these experimental results have revealed that PtN2 exhibits semiconducting property with a bandgap of ∼2 eV. This high-pressure thin-film synthesis technique could also be applied for revealing the physical properties of other novel pernitrides synthesized under ultra-high pressure, which can offer new insights into the physical properties and functionality of the pernitrides and related nitrides.

The laser-heated diamond anvil cell (LH-DAC) technique is a unique and powerful tool to access the pressure and temperature conditions corresponding to the interior of the Earth. This technique is also utilized for the synthesis of new materials having an unlikely composition, atomic coordination, or chemical bonding, which are attributed to the attractive physical properties, such as high bulk modulus and ultra-high-temperature superconductivity.1–6 

In the last two decades, high-pressure studies using the LH-DAC technique have revealed new crystal chemistry of the nitrogen-based binary compounds having a peculiar bonding between nitrogen atoms in their structure.4,5,7–10 Transition metal pernitrides (TMN2) having the N–N bonding in the structure were at first discovered in the platinum and nitrogen system at a pressure above ∼50 GPa.4,5 Platinum pernitride (PtN2) crystallizes in a pyrite-type structure and is metastable under ambient pressure.5,11 High-pressure in situ x-ray diffraction measurements yielded a bulk modulus of 372 GPa,4 which is higher than platinum (270 GPa).12,13 There have been only a few experimental studies of PtN2.4,5,11,14,15 In contrast, many theoretical calculation studies based on first-principles calculations have been performed to predict the bandgap energy, mechanical properties (elastic constants, bulk modulus, and hardness), and electronic structure.16–31 Theoretical studies demonstrate that PtN2 is a semiconductor with an indirect bandgap of ∼2 eV.11,15,17,23,25,26,29,31 However, the PtN2 crystal synthesized by using the LH-DAC is very tiny due to the limitation of the synthesis process. Therefore, detailed information on the physical properties of the experiments is mainly limited to the crystal structure and bulk modulus.4,5,11 On the other hand, the electronic structure, binding energy, and optical reflectance were experimentally determined by using a synchrotron radiation facility.14,15 Despite numerous efforts, some issues remain, such as fragility, heterogeneity, and roughness of the sample, which prevent the further reliable study of the electronic properties of PtN2.

In this study, to access profound insight into the physical properties of PtN2 synthesized by the LH-DAC, we have developed a method to synthesize a polycrystalline PtN2 thin-film on an α-Al2O3 substrate under high pressures. This method allows the characterization of the semiconducting property of PtN2 based on the current (I)–voltage (V) characteristic and optical reflectance spectrum measurements. This method is also applicable to the synthesis of thin films for the other novel nitrogen-rich compounds such as IrN2, OsN2, RuN2, TiN2, and CrN2.7,32–34

The polycrystalline platinum film was deposited by argon ion sputtering technique on the c-axis aligned single crystal of α-Al2O3 substrate (approximated size: <100 × <100 × 15 µm3) purchased from Crystal Base Ltd. [Figs. 1(a) and 1(b)]. The thickness of the polycrystalline platinum film was controlled by changing the deposition time resulting in the preparation of a platinum film with a thickness of ∼90, 200, and 300 nm in this study. The high-pressure experiments were carried out using the LH-DAC technique. The platinum-deposited α-Al2O3 substrate was located together with a ruby pressure marker in the sample chamber, which was fabricated by using a pulsed infrared laser on the indented part of a stainless-steel gasket [Fig. 1(c)]. The sample chamber was then filled with liquid nitrogen. After an increscent of the pressure to ∼50 GPa at room temperature, the sample was irradiated by the infrared laser, and the heated position was slowly moved to initiate the reaction between deposited platinum and molecular nitrogen under high pressure [Fig. 1(d)]. After heating, the laser irradiation was stopped, and the pressure was slowly decreased to ambient pressure. The recovered sample was characterized using optical microscopy, scanning electron microscopy equipped with energy dispersive spectroscopy (SEM-EDX), Raman spectroscopy, synchrotron x-ray diffraction (XRD), I–V characteristic, and optical reflectance measurements. The details of the experimental setup are described in the supplementary material.

FIG. 1.

SEM images of (a) surface and (b) cross section of the platinum film deposited on the α-Al2O3 substrate. Scale bars in (a) and (b) correspond to 3 and 1 µm, respectively. (c) Photograph of the platinum-deposited α-Al2O3 substrate located in the sample chamber. (d) Schematic illustration of the sample chamber for nitridation experiments under high pressure.

FIG. 1.

SEM images of (a) surface and (b) cross section of the platinum film deposited on the α-Al2O3 substrate. Scale bars in (a) and (b) correspond to 3 and 1 µm, respectively. (c) Photograph of the platinum-deposited α-Al2O3 substrate located in the sample chamber. (d) Schematic illustration of the sample chamber for nitridation experiments under high pressure.

Close modal

The nitridation of the platinum film with 90 nm thickness yielded few hundred nanometer-sized polygonal domains (sample No. 12, Fig. S1). The Raman spectroscopy demonstrated that this texture consists of PtN2. On the other hand, it was found that the α-Al2O3 substrate was partially exposed, which is unsuitable for the physical property measurements. Thus, the nitridation experiments were conducted by using a thicker platinum film (200 or 300 nm) deposited on the substrate. Figures 2(a)2(c) show the SEM images and corresponding Raman spectrum, respectively, of the synthesized product (sample No. 62), which was obtained from the nitridation experiments for 300 nm thickness initial platinum film. The SEM images demonstrate that the surface of the substrate is fully covered with a few hundred nanometer-sized grains. The three sharp Raman peaks below 900 cm−1 are consistent with those of PtN2, as reported in the previous studies.4,5,11

FIG. 2.

SEM images of polycrystalline PtN2 film (sample No. 62) synthesized via nitridation of the platinum film (300 nm thickness) on the α-Al2O3 substrate and the corresponding Raman spectrum measured under ambient condition. Scale bars in (a) and (b) correspond to 20 and 3 µm, respectively.

FIG. 2.

SEM images of polycrystalline PtN2 film (sample No. 62) synthesized via nitridation of the platinum film (300 nm thickness) on the α-Al2O3 substrate and the corresponding Raman spectrum measured under ambient condition. Scale bars in (a) and (b) correspond to 20 and 3 µm, respectively.

Close modal

Some of the synthesized products were further characterized by employing XRD measurements at the Nagoya University beamline BL2S1 at Aichi Synchrotron radiation facility, Aichi, Japan. The sample was attached to the tip of a polyimide capillary, and the diffracted x rays were measured while rotating the sample. Figure 3 shows the representative conventional one-dimensional (1D) XRD profiles of initial platinum and synthesized PtN2 (sample No. 62) films on the α-Al2O3 substrate. Unrolled 2-D images are also overlaid on the 1D profiles. Intense spots correspond to the diffractions from the α-Al2O3 substrate. The diffraction peaks of the initial platinum film were broad [see Fig. 3(a)]. This can be ascribed to a low crystallinity caused by the sputtering deposition. On the other hand, several sharp and intense diffraction lines were detected in addition to those of the remaining platinum and α-Al2O3 substrate after nitridation [see Fig. 3(b)]. These additional peaks were assigned to the crystal symmetry of PtN2 (Pa3̄) with a lattice parameter of a = 4.8062(2) Å, which is consistent with those of previous studies.4,5,11 The diffraction peak width of the remaining platinum was sharper, and its crystallinity was improved by thermally annealing under high pressure. Accordingly, the polycrystalline PtN2 film was successfully synthesized on the α-Al2O3 substrate via nitridation of the platinum film under high pressure. Although the unreacted platinum was detected from the XRD measurements, it was found that a thicker PtN2 film, which is suitable for the electrical resistivity and optical reflectance measurements, was synthesized under ultra-high pressure.

FIG. 3.

Representative synchrotron x-ray diffraction patterns of the (a) initial platinum and (b) synthesized polycrystalline PtN2 films (sample No. 62, nitridation of platinum film with 300 nm thickness) on the α-Al2O3 substrate. Upper and lower overlaid images on the conventional one-dimensional profiles correspond to the unrolled XRD images (90° of azimuth angles). The Debye rings labeled with PtN2 and Pt correspond to the PtN2 and remaining platinum, respectively. Strong spots are derived from the α-Al2O3 substrate.

FIG. 3.

Representative synchrotron x-ray diffraction patterns of the (a) initial platinum and (b) synthesized polycrystalline PtN2 films (sample No. 62, nitridation of platinum film with 300 nm thickness) on the α-Al2O3 substrate. Upper and lower overlaid images on the conventional one-dimensional profiles correspond to the unrolled XRD images (90° of azimuth angles). The Debye rings labeled with PtN2 and Pt correspond to the PtN2 and remaining platinum, respectively. Strong spots are derived from the α-Al2O3 substrate.

Close modal

Figure 4 shows the I–V characteristic for the α-Al2O3 substrate, initial platinum film, and synthesized PtN2 (sample No. 70) on the substrate. α-Al2O3 substrate shows the typical I–V characteristic of an insulator, while the linear I–V characteristic (Ohm’s law) is observed for the platinum film on the α-Al2O3 substrate. The electrical resistivity of the platinum film was slightly higher than the reference value (this study: ∼3.2 × 10−6 Ω‧m, Ref. 35: 1.1 × 10−7 Ω‧m). This was probably due to a poor crystallinity via sputtering deposition, which can also be recognized from the broad x-ray diffraction peaks of platinum, as shown in Fig. 3. On the other hand, as shown in Fig. 4, the I–V characteristic for the PtN2 film measured with the two-terminal method (E2 and E3) demonstrates that current increases drastically with the increase in the applied voltage. This I–V characteristic was also observed for the other samples (sample Nos. 52 and 57, Fig. S3) and is different from the behaviors of platinum and α-Al2O3. The XRD measurements demonstrated that initial platinum remained together with synthesized PtN2. If the residual platinum was contacted with each other or electrodes, the electrical conduction pass is formed, and the linear I–V characteristic is probably observed as well as the case of measuring only the initial platinum film. Therefore, the observed I–V characteristic for the high-pressure synthesized product is derived from PtN2 and this can be recognized as typical of a Schottky barrier effect36–38 at the electrode metal (Au)–semiconductor (PtN2) junction. The origin of the Schottky barrier junction is probably due to the large difference between the Fermi level of the Au electrode and the conduction band level of the PtN2 semiconductor. Accordingly, the present result experimentally demonstrates that the PtN2 film exhibits semiconducting property, and changing the electrode metal might reveal further electrical properties.

FIG. 4.

Schematic illustration (a) and optical photograph (b) of measurements, and I–V characteristic (c) for α-Al2O3 substrate, initial platinum, and PtN2 (sample No. 70) synthesized on the α-Al2O3 substrate. Ei (i = 1–4) in the inset figure corresponds to the electrodes. The sample was fixed with the other probe when the I–V characteristic was measured (see Fig. S2). The I–V characteristic of platinum was measured with the four-terminal method (E1 and E4 for I, E2 and E3 for Vs), while PtN2 and α-Al2O3 were measured with the two-terminal method (E2 and E3).

FIG. 4.

Schematic illustration (a) and optical photograph (b) of measurements, and I–V characteristic (c) for α-Al2O3 substrate, initial platinum, and PtN2 (sample No. 70) synthesized on the α-Al2O3 substrate. Ei (i = 1–4) in the inset figure corresponds to the electrodes. The sample was fixed with the other probe when the I–V characteristic was measured (see Fig. S2). The I–V characteristic of platinum was measured with the four-terminal method (E1 and E4 for I, E2 and E3 for Vs), while PtN2 and α-Al2O3 were measured with the two-terminal method (E2 and E3).

Close modal

The optical reflectance spectra were measured between wavelengths of 215 nm and 15 µm. First, the optical reflectance spectra were measured with a vertical incident optical geometry (Fig. S4). In this configuration, strong reflection was detected, particularly in the low-energy regions. This is probably either due to the interference between the α-Al2O3 substrate surface and the backside or from the residual platinum. Therefore, to avoid the specular reflection component and collect the diffuse one, the reflection spectrum was measured with a 40° or more tilting angle of the sample surface to the vertical incident axis of the objective mirror. As shown in Fig. 5, no reflected lights were detected from the initial platinum film because incident lights were reflected out of the objective mirror. For PtN2 film (sample No. 70), the characteristic profile with two broad peaks (3 and 5 eV) was observed along with an increase in the background, although the reflected lights were weak due to the scattering from the randomly oriented aggregation of the PtN2 grains on the substrate. Similar spectra were observed for the other synthesized PtN2 films (sample Nos. 73 and 75, Fig. S4). According to the Drude–Lorentz model, the increase in the reflectance corresponds to the inter-band optical transition. To evaluate the relation between optical response and electronic structure, the density of state (DOS), absorption, and reflectance spectra were calculated based on the first-principles methods. The details of the calculation are described in the supplementary material. The band diagram and DOS, calculated by using a modified Becke–Johnson function,39,40 demonstrate that PtN2 shows the indirect transition semiconductor with a bandgap energy of 1.6 eV (Fig. S5), consistent with previous studies.11,15,17,23,25,26,29,31 Density functional theory (DFT) calculations also show a drastic increase in the absorption at the photon energy corresponding to the bandgap (Fig. S6, lower). The calculated reflectance spectrum is similar with the measured reflectance profile, especially regarding a few broad peaks below 6 eV, although the experimental spectrum lies higher in photon energy. The comparison of experimental and theoretical profiles strongly suggests that the bandgap energy of the PtN2 film is likely to lie at ∼2 eV.

FIG. 5.

Representative optical reflectance spectra of initial platinum and PtN2 (sample No. 70) synthesized on the α-Al2O3 substrate. Spectra were measured from two different positions, and both are displayed. The simulated reflectance spectrum of PtN2 is also shown with a dashed line together with calculated bandgap energy of 1.6 eV (vertical dashed line).

FIG. 5.

Representative optical reflectance spectra of initial platinum and PtN2 (sample No. 70) synthesized on the α-Al2O3 substrate. Spectra were measured from two different positions, and both are displayed. The simulated reflectance spectrum of PtN2 is also shown with a dashed line together with calculated bandgap energy of 1.6 eV (vertical dashed line).

Close modal

The present method allowed to synthesize polycrystalline thin films and reveal the semiconducting properties of PtN2. Moreover, polycrystalline pernitride thin films would be utilized to understand the other physical and mechanical properties. On the other hand, this technique can also be used for other transition metal pernitrides, related nitrogen-rich nitrides, and multicomponent systems. The bandgap of semiconductors is often controlled by the solid solution among the isostructural compounds. Recent high-pressure studies for binary nitrides found that some elements adopt pernitrides with pyrite-, marcasite-, and arsenopyrite-type crystal structures. The bandgap energy of pernitrides has not yet been experimentally clarified, while theoretical calculation suggests that the bandgap energy systematically depends on their lattice parameters, as shown in Fig. S7 for the example of pyrite-type pernitrides. Thus, this method is also applicable to synthesize the multicomponent pernitrides and reveal the electronic properties if a solid solution among the different nitrides is formed on the substrate under high pressure.

The nitridation of a platinum thin-film under ultra-high pressure using the LH-DAC succeeded in the synthesis of a polycrystalline PtN2 thin-film on a α-Al2O3 substrate. The electrical resistivity and optical reflectance measurements as well as the first-principles calculation found that PtN2 exhibits a semiconducting property with a bandgap of ∼2 eV. Our developed method can also be utilized with new pernitrides that can be synthesized at high pressure and recovered under ambient condition.

See the supplementary material for the details of experimental setup and calculations, results of SEM observation, I–V measurements, optical reflectance measurements, calculated band diagram and density of states, calculated absorption and reflectance spectra, and the pressure dependence of bandgap for the pyrite-type pernitrides.

The synchrotron XRD experiments were conducted at the BL2S1 of Aichi Synchrotron Radiation Center, Aichi Science & Technology Foundation, Aichi, Japan (Proposal Nos. 2019N4001, 2019N5001, and 2019N6002) with the financial support of Synchrotron Radiation Research Center, Nagoya University. The authors would like to thank Dr. T. Nagae (Nagoya University) and Professor N. Watanabe (Nagoya University) for their technical support with the synchrotron XRD measurements. We are very grateful to Dr. T. Hatano and Professor K. Soda for technical support and discussions. This work was supported by JSPS KAKENHI Grant Nos. 19H05790, 20K21080, and 21H04615 and researcher exchange program.

The authors have no conflicts to disclose.

The data that support the findings of this study are available within the article and its supplementary material.

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