Room-Temperature Synthesis of Titanium Nitride Using Metastable Nitrogen

Abstract

The room-temperature synthesis of titanium nitride (TiN) is presented using the reaction of metastable nitrogen (MSN) with a titanium metal surface. The MSN is generated in a nitrogen glow discharge with plasma-ion filtering using a commercial direct analysis in real time (DART) source. The MSN is flowed over a titanium substrate at ambient pressure producing TiN surfaces that are ultra-clean and suitable for plasmonic applications. This is demonstrated using surface-enhanced infrared absorption spectroscopy (SEIRA), producing a 100-fold signal enhancement. Nitriding using MSN could find general applications in producing nitrided surfaces with small-scale structures.

1. Introduction

Titanium nitride (TiN) coatings have a broad range of applications that are based on useful properties such as hardness, corrosion resistance, thermal stability and excellent wear resistance [1,2,3,4,5]. In general, TiN is deposited on surfaces by physical vapor deposition (PVD) evaporation, sputtering, ion plating and chemical vapor deposition (CVD). Thin coatings of TiN, typically less than 5 microns, deliver hardness and wear resistance for tools and mechanical components. TiN is non-toxic and it provides biocompatibility and long life to medical implants. TiN provides a barrier to diffusion between the implant’s structural material and living tissue [6]. The TiN surface has a low affinity to organic contamination. This has led to its use on Mars sample return missions to provide in an inert, ultra-clean container that is resistant to accumulating chemisorbed contamination [7].

The optical properties of TiN are also the basis of a growing number of applications. This is evident in the fields of plasmonics, nonlinear optics, nanophotonics, near-field microscopy and surface-enhanced spectroscopy [8,9,10,11,12,13,14]. These diverse applications depend on precise control of the optical nanostructures. For plasmonic applications, free electrons in plasmonic materials collectively couple to an applied electromagnetic field when excited by light. The nanometer-scale structures need to be maintained in order to produce optical resonance conditions that provide a local concentration of the field. Gold is often selected as a plasmonic material because of its excellent plasmonic properties and its chemical stability [15]. This is the basis of surface-enhanced spectroscopies. However, gold is a soft material with high metal diffusion at elevated temperatures. Thermally induced microstructural variation causes changes in the optical constant and may degrade the plasmonic structure.

The thermal stability of TiN is particularly useful under the intense confined local electric fields encountered in near-field optical applications. With its high melting point (>2900 °C), TiN can sustain high temperatures while still maintaining large optical nonlinearities for high-power nonlinear plasmonics [16]. Titanium nitride (TiN) has been studied as a plasmonic material that overcomes many of the limitations of conventional plasmonic metals. TiN shows comparable linear plasmonic performances in the infrared region with respect to gold and silver [16].

In this work, TiN was synthesized at room temperature under ambient pressures by exposing titanium metal to a stream of metastable nitrogen gas produced by a commercial direct analysis in real time (DART) source. DART was originally designed to provide soft ionization for mass spectroscopy applications [17]. The DART system provides a simple, temperature-controlled source of metastable nitrogen. A TiN surface coating on titanium metal was produced by simply exposing the metal at room temperature (30 °C) in front of the source nozzle. The formation of TiN on the surface was directly confirmed by Raman spectroscopy and infrared spectroscopy. The resulting TiN substrate produced surface-enhanced infrared absorption (SEIRA). This is a demonstration of its surface plasmonic properties and as further confirmation of the TiN synthesis.

2. Materials and Methods

A schematic of the commercial DART source used in this study is shown in Figure 1. Nitrogen enters a tube where a DC point-to-plane glow discharge is created using an electric potential in the range of 3–4 kV [18,19,20,21]. This flows into a second chamber that contains intermediate and exit electrodes. The ionized species are filtered from exiting by using a bias applied to the discharge needle and the intermediate electrodes. The exit gas contains nitrogen with various neutral, vibronically excited-state nitrogen molecules [22,23,24]. This includes the ground-state N(4S), metastable N(2D) and N(2P) atoms. These recombine to produce a range of excited N2 in states with excess energy ranging from 6.2 eV up to 12.3 eV.

The TiN films produced in this study use titanium metal, (type 2 sheet, www.onlinemetals.com (accessed on 26 June 2022)) exposed to metastable nitrogen from a DART-100 source (Ionsence, Saugus, MA, USA). The DART parameters were set to a needle voltage of 3500 V and 6.3 mA. The discharge electrode was set at 250 V and the exit grid at 250 V. The exit temperature is controlled by the DART-100 from room temperature up to 500 °C. The titanium target was placed directly in front of the DART source exit nozzle at a distance of approximately 1 mm. The nitrogen flow rate was set at 1 L per minute. The temperature was set to 30 °C and the run duration was 5 h. The flow was sufficient to keep the target surface purged and prevent exposure to the air.

The Raman analysis used a Bruker Senterria Raman microscope (Bruker Optics, Ettlingen, Germany) equipped with a 20 mW, 532-nanometer wavelength, excitation laser. The spectra were acquired with 5 s accumulation time and 10 coadditions. The infrared reflectance analysis used a BioRad FTS 6000 FTIR spectrometer (Agilent, Santa Clara, CA, USA) equipped with a diffuse reflectance attachment (Pike Technologies, Fitchburg, WI, USA). The total reflectance was measured relative to clean reference titanium.

The SEIRA analysis used a Bruker 80 V FTIR microscope (Bruker Optics, Ettlingen, Germany) equipped with an attenuated total reflectance attachment (ATR) using a diamond optical element. The SEIRA test analyte was diisononyl phthalate (Aldrich). This was cast as a thin film using the ‘‘drop-drying method’’ from a dilute methanol solution that is deposited with a micro-syringe. The drop-drying method has been previously shown to produce a uniform thin film, provided the solvent wets the substrate without beading [25]. The TiN-coated substrate was pressed against the diamond ATR optical element. The SEIRA was measured relative to a clean, reference titanium surface.

3. Results

Infrared spectroscopy provides confirmation of the TiN film formation after a 5 h exposure to metastable nitrogen at 30 °C. The infrared reflectance spectrum of the TiN film is shown Figure 2. TiN is featureless through most of the mid-infrared, with highly characteristic Ti–N stretching bands at 418 cm⁻¹ and 447 cm⁻¹ that can be attributed to the formation of N–Ti–N bonds. The strong band at 447 cm⁻¹ is the IR active zone center optic phonon transverse optical (TO) mode. There are weak longitudinal (LO) mode peaks extending up to 800 cm⁻¹ [26]. The infrared spectrum confirms the TiN did not have a significant amount of oxidation or side products being formed during the synthesis.

The Raman spectrum, shown in Figure 3, reveals the characteristic TiN bands. The main features of the Raman peaks are related to both acoustic and optical phonons, with predominate first-order modes and weaker second-order modes. The first-order phonons include the transverse acoustic (TA), the longitudinal acoustic (LA), the transverse optical (TO) and the longitudinal optical (LO) modes. The weaker Raman modes labeled 2A, A + O and 2O are related to second-order scattering [27]. The Raman peaks are sensitive to lattice defects and strain. The frequency shift of the first-order acoustic and optical bands has been used as an indicator of the N defect concentration, which is related to the Ti:N ratio. Here, the resulting film of TiNx has an x value of approximately 0.92. This is based on shift of the transverse acoustic (TA) mode positioned at 219 cm⁻¹ [28].

The plasmonic resonance properties of the TiN film is demonstrated by measuring the surface-enhanced infrared absorption (SEIRA). SEIRA was tested by casting a thin 1-nanometer analyte film (diisononyl phthalate) on the TiN surface and a bare titanium reference surface. Figure 4 shows the SEIRA spectrum of a 1-nanometer analyte film compared to an analyte film cast on a bare titanium surface. The analyte cast on bare titanium is undetectable. From this, the SEIRA enhancement factor is estimated to be approximately 100 for the TiN. The enhancement factor is calculated from the ratio of the signal strength of the SEIRA and the signal-to-noise ratio of the unenhanced film.

4. Discussion

In this paper, the significance of metastable-species activity for the process of surface nitriding has been demonstrated. This is consistent with the strong influence of neutral metastable species on ionization growth in nitrogen discharges due to the role for highly vibrationally excited states [29]. The exposure of titanium metal to metastable nitrogen at ambient pressures near room temperature for several hours produced a thin TiN film. The thickness is approximately 40 nanometers based on the plasmonic nanostructures required to produce SEIRA and the strength of the infrared absorption [30].

The TiN substrate synthesized in this study is not optimized for maximum SEIRA. In this case, SEIRA relies on the native roughness of the substrate coupon to confine surface plasmons and mediate plasmon resonance. Larger enhancements are expected if the surface morphology is tuned to control the surface plasmon resonance. The plasmon resonance can be controlled by adjusting the substrate precursor’s morphology or by using engineered nanostructures or nanoparticles. The work presented here suggests a number of other parameters that can be adjusted to control the rate and stoichiometry of the nitride formation. In this work, the lowest temperature (30 °C) that could be controlled was selected. The metastable nitrogen was produced with an ion-grid filter to reduce nitrogen plasma. Future work could evaluate parameters such as temperature and adjust the level of nitrogen plasma that is mixed with the MSN.

5. Conclusions

The synthesis of TiN using the DART metastable nitrogen source provides a thin controlled layer of TiN suitable for plasmonic applications. The TiN film formation on the titanium metal was confirmed by both infrared reflectance spectroscopy and Raman spectroscopy. Indirect confirmation for the presence of TiN is the gold color and demonstration that the TiN film supports surface plasmon resonance needed for SEIRA. The low-temperature nitriding of metal surfaces using metastable nitrogen has several advantages. It retains the base substrate’s fine structure and does not require vacuum processing.

The ability to produce TiN surfaces that are very clean is useful for SEIRA and other plasmonic applications. The surface cleaning may be confirmed by using the DART source to simultaneously nitride and monitor the surface contamination using mass spectrometry. The ultra-clean TiN coatings have low contamination affinity and provide an inert surface with reduced accumulation of organic contamination. TiN inhibits the chemical absorption of reactive, low-molecular-weight molecules [7].

The use of a DART-like MSN source for the synthesis of TiN suggests a general route for the synthesis of nitrides. Nitrides have not been studied as extensively as oxides, making them open to novel materials discovery. Of recent interest are nitrogen-rich nitrides which possess potentially useful semiconducting properties for electronic and optoelectronic applications [31].