Colloidally synthesized and bandgap-engineered luminescent titanium nitride quantum dots

Abstract

Semiconductor nanomaterials, such as cadmium, lead, and mercury chalcogenides, as well as lead halide perovskites, exhibit excellent optical, electronic, photonic, and photovoltaic properties, making them promising for applications in solar cells, LEDs, and X-ray photodetectors. However, heavy metals in these nanomaterials raise concerns about their use in devices and the recycling and disposal of such devices. Therefore, developing greener luminescent materials is crucial for sustainable optoelectronic and photovoltaic technologies. We report a colloidal chemical method for engineering brilliantly luminescent titanium nitride (TiN) quantum dots showing tunable optical bandgap (1.8–2.2 eV) and multicolor photoluminescence. We demonstrate the TiN quantum dot structure and properties using HRTEM, SEM-EDX, XRD, XPS, Raman spectroscopy, and steady-state and time-resolved fluorescence spectroscopy, confirming their size, morphology, chemical composition, crystalline structure, bandgap, and luminescence properties. This research presents luminescent TiN quantum dots as promising substitutes for metal chalcogenides and lead halide perovskites in sustainable electrooptical and photovoltaic technologies.

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1. Introduction

The most attractive luminescent semiconductor nanocrystals (NCs) and quantum dots (QDs) belong to lead halide perovskites and metal chalcogenides.^1–3 These tiny crystals show exceptional optical and electronic properties, positioning them at the forefront of photovoltaic and electro-optical technologies.^4,5 However, toxic heavy metals such as Cd, Pb, and Hg often raise environmental and occupational safety concerns, limiting their expansion to consumer products. Therefore, synthesizing safe and green luminescent semiconductors free from toxic elements has become an emergent research subject.⁶

Driven by the need for environmentally benign alternatives to lead halide perovskites and heavy metal chalcogenides, significant progress has been made in developing various nanomaterials with excellent optical and electronic properties.⁶ However, most heavy metal-free nanomaterials face challenges such as complex synthesis routes and conditions, poor scalability, and insufficient stability. Among emerging candidates, metal nitrides and phosphides are receiving increasing attention due to their intrinsic stability, non-toxic nature, and promising physicochemical, optical, catalytic, and electronic properties,^7,8 positioning them as versatile alternatives to Cd-, Pb-, and Hg-based materials from both fundamental scientific and sustainable technological perspectives. Notably, titanium nitride (TiN, hereafter referred to as TN) holds a special place due to its chemical stability, thermal and electrical conductivity, hardness, and corrosion resistance, in addition to its non-toxic nature, making it attractive for applications in microelectronics, jewelry, coatings, and cutting tools.^9–14 Also, TN shows plasmonic properties,^9,10,15–20 like noble metal nanoparticles. Nevertheless, the synthesis of bulk, thin-film, or nanocrystalline TN has been challenging, requiring sophisticated chemical processing techniques and drastic experimental conditions. Furthermore, luminescent and bandgap-engineered TN NCs and QDs are yet to be established.

High-temperature gas-, plasma-, and solid-phase synthesis has been the standard method for preparing TN in various morphological forms.^21–23 For example, Saeki et al. synthesized nanoscale TN by a high-temperature (400–1000 °C) gas-phase reaction between TiCl₄ and NH₃.²¹ Additionally, Karaballi et al. prepared TN nanoparticles (NPs) from TiO₂ and Mg₃N₂ powders at 1000 °C over 12 h, showing near-infrared (ca. 720 nm) surface plasmon resonance (SPR).¹⁵ Other synthesis routes include plasma-assisted synthesis of plasmonic TN NPs,^9,24 high-temperature (800–1400 °C) nitridation of TiO₂ using ammonia,²² or high-pressure (50 MPa) ball milling of TiO₂ with carbon black and nitrogen.²³ These methods produce conducting TN films or plasmonic TN NPs. However, in 1954, Munster et al. observed the semiconducting behavior of TN thin films deposited on a quartz substrate, which challenged the concept of TN being purely metallic.²⁵ Sixty years later, Solovan et al. demonstrated the n-type semiconducting behavior of covalent TN prepared using magnetron sputtering Ti in an N₂ atmosphere at 570 K, providing 3.4 eV direct bandgap thin films.²⁶ Recently, Lu et al. prepared atomically thin TN films by high-temperature topochemical nitridation of titania, demonstrating the n-type semiconducting behavior, with a 3.56 eV bandgap.²⁷

Despite the plasmonic properties demonstrated by TN films and NPs, TN clusters synthesized by an ion plasma method,²⁸ and mesoscale titanium oxynitride synthesized in a high-temperature furnace²⁹ exhibited visible and near-infrared (NIR) emissions, leaving the synthesis of TN QDs with tunable emission an open challenge. We accomplished the colloidal synthesis of TN QDs with green-yellow-orange-red emission. These QDs exhibit a rock salt-type face-centered cubic lattice (Fm3̄m space group), as determined by high-resolution STEM and X-ray diffraction analyses. The colloidal TN QD solution shows a stable emission for several weeks. We demonstrate the morphological, structural, and optical properties of these QDs, which exhibit broad optical absorption and tunable emission, can be promising for environmentally friendly solar cells and LEDs.

2. Results and discussion

We characterized the size distribution and lattice parameters of TN QDs using STEM and HRTEM images [Fig. 1a (i, iii) and S1 (a, b, d)]. The QD size distribution (Fig. 1b) was determined by analyzing approximately 200 single QDs in the STEM images [Fig. 1a (iii) and S1 (d), yielding an average size of 2.5 nm. While the fast Fourier transform (FFT) patterns [Fig. 1a (ii) and S1(c)] are consistent with the cubic lattice (inset of Fig. 1b) predicted from the PXRD data (Fig. 2a), the HRTEM images [Fig. 1a (i) and S1(a and b)] verify the interplanar distance at 3.0 Å. Fig. 1a (iv, v) shows the photographs of a colloidal QD solution in room light and under UV light. We examined the absorption and PL spectra (Fig. 1c) of the QD sample to understand its optical properties. The broad UV-vis absorption spectrum is consistent with the red emission and colloidal nature of the sample. We estimated the average bandgap at 1.8 eV by fitting the absorption edge. The corresponding Tauc Plot is shown in Fig. S2. The PL spectrum showed an intensity maximum at 676 nm, with a spectral half-width of 85 nm (637–722 nm).

Fig. 1 Structural and optical properties of TN QDs. (a) A STEM image, a FFT pattern, and optical images of TN QD samples: (i) an HRTEM image of a single QD (image size is 15 nm × 15 nm), (ii) the FFT pattern of the QD in ‘i’, (iii) a large area STEM image of the QDs, (iv and v) the photographs of a TN QD colloidal solution (iv) in room light and (v) under 365 nm light, (b) TN QD size distribution from image j analysis of STEM images (inset: the structure of TiN assigned from HRTEM analysis), and (c) (i) absorption and (ii) PL spectra (λ_ex = 400 nm) of a colloidal TN QD solution in toluene.

Fig. 2 (a) PXRD patterns of (i) the TN QD and (ii) a commercial bulk TN powder samples. (b) SEM-EDS elemental maps corresponding to (i and iv) Ti, (ii and v) N, and (iii and vi) Ti–N overlay for (i–iii) commercial TN, and (iv–vi) the as-synthesized colloidal TN QDs. The scale bars are 100 (i–iii) and 60 (iv–vi) μm.

These QDs show characteristic PXRD peaks [Fig. 2a(i)] at 36.62°, 42.65°, and 61.8°, corresponding to the (111), (200), and (220) planes, respectively, showing the rock salt-type face- centered cubic (FCC) lattice with Fm3̄m space group, like bulk TN [Fig. 2a(ii)].^9,10,30 The peaks at 31.72° and 45.42° are assigned to the byproduct NaCl. The elemental composition of the TN QDs is characterized using SEM-EDX. Elemental maps of the QD [Fig. 2b (iv–vi)] and the commercial samples [Fig. 2b (i–iii)] show a homogeneous distribution of Ti and N. The corresponding SEM images are in the SI (Fig. S3a and b).

To validate the chemical bonding of the prepared TN QDs, we conducted XPS measurements and compared them with those of the commercial bulk sample. The XPS survey spectra of both TN QD and the commercial bulk sample (Fig. S4) showed characteristic N 1s and Ti 2p peaks at 397 and 458 eV, respectively. The TN QDs and the commercial TN sample showed similar high-resolution XPS spectra of Ti 2p (Fig. 3a and b) and N1s (Fig. 3c and d) with characteristic Ti 2p_3/2 (454.9 eV) and Ti 2p_1/2 (461.0 eV) peaks,^10,31,32 which is at 397.0 eV for N1s (Fig. 3c and d),^10,31 confirming the TN structure. Nevertheless, partial oxidation or reaction of the samples with the FTO substrate is apparent from the Ti–O–N (456.4 eV), Ti–O (458.2 and 463.8 eV), TiO_xN_y (397.8), and N–O (400.3 eV) peaks. Fluorine-doped tin oxide (FTO) coated glass plates were used as the sample substrate to suppress charge-up effects. Both samples showed Ti–O and Ti–O–N peaks, suggesting the oxidation by an X-ray-induced reaction between samples and the FTO-coating, which is further supported by the absence of PXRD peaks corresponding to TiO₂ in both samples. Additionally, we employed Raman spectroscopy to understand the structure of these QDs. The vibrational fingerprints ca. 220, 345, 445, and 565 cm⁻¹ for TN QDs (Fig. 3e, which are ca. 210, 305, 450, and 540 cm⁻¹ for commercial TN, Fig. S5), corresponding to the transverse acoustic (TA, ca. 210 cm⁻¹), longitudinal acoustic (LA, ca. 345 cm⁻¹), the second order (2A, 445 cm⁻¹) and, longitudinal optical (LO, ca. 565 cm⁻¹) phonons are consistent with the corresponding modes for bulk TN.³³ However, the Raman modes (Fig. 3f) of the as-synthesized and commercial samples under high-intensity laser irradiation in air undergo oxidation, similar to bulk samples.¹¹ We avoided oxidation of the samples by sealing the samples in an Argon chamber. Both samples showed the characteristic Raman signals without any signal corresponding to titanium oxide. Overall, the PXRD, HRTEM, SEM-EDS, XPS, and Raman data confirm the as-synthesized TN sample by comparing the data with a commercial sample and the literature.

Fig. 3 (a–d) High-resolution XPS spectra corresponding to (a and b) Ti 2p, (c and d) N 1s, for (a and c) the TN QD and (b and d) the commercial bulk TN samples. The background in the XPS spectra was estimated using the Shirley method. (e and f) Raman spectra of (e) TN QDs in an Ar atmosphere, and (f) (red) TN QDs and (black) commercial bulk TN in air.

Like bulk crystals and films, commercial microcrystals are non-luminescent. Additionally, high-temperature-synthesized TN NPs, produced by both solid-state and plasma methods, are generally plasmonic.^10,15 Conversely, the as-synthesized colloidal sample showed intense red emission [Fig. 1a (iv)], demonstrating an engineered optical bandgap for ligand-capped TN QDs. The PL spectra recorded at 20 nm excitation intervals – from 400 to 580 nm – (Fig. S6a) showed identical spectral shape and width, with a slight (<10 nm) redshift in the PL intensity maximum. Also, we recorded the PL excitation (PLE) spectra for 725, 700, 675, and 650 nm (Fig. S6b) wavelengths, showing identical spectra. The PLE spectral shape was independent of the excitation wavelengths, confirming the red-emitting QDs as the major component in the colloidal TN sample.

A faint tailing on the higher energy side of the PL spectrum [Fig. 1c (ii)] attracted us to investigate the presence of higher bandgap QDs in the sample. Therefore, we drop-cast a colloidal TN solution on a glass coverslip and examined the PL image and spectra of individual particles in the sample using a microspectrometer. Although the PL image (Fig. 4a) and spectrum [Fig. 1c (ii)] of this sample recorded under 404 nm laser excitation showed predominantly red emission (ca. 675 nm), we identified green-, yellow-, orange-, and red-emitting particles in the sample microscopically and spectroscopically (Fig. 4a and b). PL spectra of individual color-emitting QDs were collected using a fiber spectrometer, demonstrating PL maxima ca. 570, 600, and 615 nm, which substantiate the high-energy tail in the PL spectrum of the QD solution. These multiple emission maxima, along with the QD size distribution (Fig. 1b) obtained by STEM imaging, indicate a quantum confinement effect. A PL image of isolated TN QDs is shown in Fig. 4c, which was captured using an electron multiplying charge-coupled device (EM-CCD) camera. Despite the different PL spectral maxima, the narrow size distribution and the small fraction of QDs with wider bandgaps prevented us from physically separating QDs with individual emission colors, leaving size-controlled colloidal synthesis of TN NCs and QDs an open challenge. Although Mainet et al. reported 400–500 nm-emitting TN nanoclusters by ionized cluster beam deposition,²⁸ the present work introduces a colloidal method for synthesizing highly luminescent multicolor TN QDs, opening a corridor for transitioning from heavy metal-based QDs and NCs to environmentally benign alternatives. To characterize the exciton recombination dynamics in isolated TN QDs, we recorded energy-dependent PL decays of TN QDs prepared by drop-casting a sub-nanomolar colloidal solution (Fig. 4d) using a time-correlated single-photon counting (TCSPC) system. The isolated QDs were excited with a picosecond laser (405 nm, 75 ps, 500 kHz). The emitted photons were collected using a combination of an objective lens (100×, NA = 0.95), a long-pass (LP), or band-pass (BP) filter, an avalanche photodiode (SPCM-AQR14), and a single-photon counting module (SPC 830). PL decays were collected using a 480 nm long-pass (LP), 515–550 nm band-pass (BP), 545–580 nm BP, 573–613 nm BP, 637–662 nm BP, or 700 nm LP filter. The highlighted areas in the PL spectrum (Fig. 4d, inset) represent the energy bands corresponding to the PL decays. These decays were fitted using the 2^nd- or 3^rd-order exponential equation. The fast decay component, τ₁ suggests excitonic recombination characteristics for the small-sized quantum dots. In contrast, the slow decay components, τ₂ and τ₃ suggest phonon- and trap-assisted delayed recombinations. The TN QD should have dangling bonds related to undercoordinated Ti atoms. These nitrogen vacancy–induced defect states on the TiN surface lead to nonradiative recombination. The decay constituted by all the photons (480 nm LP) provided an average lifetime of 4.6 ns. Similar PL lifetimes were obtained for the photons in the 515–550 (τ_av = 4.8 ns), 545–580 (τ_av = 4.8 ns), 573–613 (τ_av = 4.5 ns), 637–662 (τ_av = 4.2 ns), and >700 (τ_av = 4.2 ns) nm ranges, suggesting identical electron and hole states, including for the small fractions of high-energy excitons corresponding to the short-wavelength tail in the PL spectra. Conversely, the 39.3 ns decay time collected using the 700 nm LP filter suggests emission from larger QDs with weakly confined excitons or a contribution from trap-assisted delayed recombination. Oleylamine can coordinate to surface Ti atoms, suppressing the dangling bonds and nonradiative decays. As a result, the PL quantum yield increased from below 1% to 2.8%.

Fig. 4 (a) PL image (λ_ex = 404 nm) of TN QDs drop-casted on a glass coverslip (scale bar: 25 μm) acquired through a 480 nm LP filter. (b) PL spectra of isolated TN QD aggregates with different PL colors and spectral maxima (λ_ex = 404 nm) acquired through a 480 nm LP filter. The inset shows the corresponding PL images of the aggregates (each image size is 5 μm × 5 μm). (c) A single-particle PL image of isolated TN QDs on a glass coverslip (scale bar: 10 μm) acquired through a 480 nm LP filter. (d) PL decays of isolated TN QDs acquired through different emission filters, inset: the corresponding PL spectra of the solution, indicating different emission filters.

We correlate the experimental optical bandgap with the electronic structure of QDs. While the colloidally synthesized QDs demonstrated brilliant PL, free electrons in the covalent Ti–N framework, and the high dielectric constant render a zero-bandgap to bulk TN,³⁴ suggesting quantum confinement energy alone is insufficient to account for the PL spectra with different maxima (2.18, 2.07, and 2.02 eV). Conversely, the n-type semiconducting nature of TN NPs and thickness-dependent metallic-to-semiconductor transition properties of TN films have often been assigned to an oxygen-induced Fermi energy shifting midway to the titanium d- and nitrogen p-bands, analogous to the bandgap originating from hybridized atomic orbitals of the metal kernel and d → sp transitions in luminescent gold clusters.³⁵ Nonetheless, the as-synthesized colloidal TN QDs retain intact and intense PL under an argon atmosphere, ruling out oxygen termination. Therefore, we hypothesize that a combined effect of hybridized Ti–N orbitals, ligand orbitals, and dielectrically confined charge carriers contributes to the optical bandgap and PL of the TN QDs, underscoring the importance of evaluating quantum confinement.

3. Conclusions

We report the first colloidal synthesis of luminescent TN QDs and demonstrate their structure and properties using HRTEM, SEM-EDX, XRD, XPS, Raman spectroscopy, fluorescence spectroscopy, and TCSPC, confirming their size, morphology, chemical composition, crystalline structure, bandgap, and luminescence properties. While the colloidal sample is predominantly red-emitting (676 nm), microspectroscopic results revealed the presence of green-, yellow-, and orange-emitting particles. These findings suggest TN QDs with tunable bandgap can be promising alternatives to conventional heavy metal chalcogenide and halide perovskite QDs for sustainable optoelectronic and photovoltaic devices.

4. Experimental

4.1. Chemicals

The following reagents, solvents, and materials were used as received in the synthesis, purification, characterization, and studies of TN QD samples: titanium(IV) chloride (TiCl₄, 1 M solution in toluene, TCI Chemicals), sodium amide (NaNH₂, Sigma–Aldrich), n-hexadecane (>98.0%, TCI Chemicals), oleyl amine (OAm, TCI Chemicals), toluene (Wako), cresyl violet perchlorate (TOP, Sigma–Aldrich), and tri-N-octylphosphine (TOP, Sigma–Aldrich).

4.2. Synthesis of TN quantum dots

We demonstrate the ligand-assisted colloidal synthesis of highly luminescent TN QDs using titanium amide (Scheme 1) obtained by a room temperature reaction between titanium tetrachloride (TiCl₄) and sodium amide (NaNH₂). Briefly, we mixed a 2.5 mL TiCl₄ solution (1 M, toluene) with 507 mg (13 mmol) NaNH₂ in an argon-filled 100 mL flask. The mixture was heated to 90 °C and maintained at this temperature under continuous argon flow and vigorous stirring for 90 minutes, yielding a golden yellow solution. Next, toluene was removed under a vacuum, and 20 mL hexadecane was added. The mixture was immersed in a silicon oil bath at 280 °C and maintained at this temperature for 30 min, which provided a dark-brown TN solution. We arrested the TN crystal growth by injecting 50 μL of oleyl amine while cooling the solution to room temperature. The TN QDs were isolated and purified by centrifugation at 10 000 rpm, after the addition of 50 μL TOP.

Scheme 1 The reaction scheme of TN QD synthesis.

4.3. Characterization

We characterized the TN quantum dots and compared their elemental composition and structural properties with those of commercial bulk TN. Specifically, we employed powder X-ray diffraction (PXRD), scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDS), high-resolution transmission electron microscopy (HRTEM), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). Details of the synthesis, purification, and characterization of luminescent TN QDs are in the SI. Further details are provided in the SI.

Author contributions

V. B. conceived the project. A. M. synthesized and purified the TiN QDs. M. K. and C. S. conducted the XPS experiments. M. F. K. and Y. M. conducted the Raman experiments. A. M. and T. O. performed the SEM and EDS analyses. A. M. and V. B. performed the STEM and TEM experiments. A. M. and V. B. conducted the fluorescence microscopy and spectroscopy experiments. T. O. conducted the XRD experiments. A. M. wrote the manuscript draft with inputs from all authors. V. B. and T. O. finalized the manuscript. All authors contributed to manuscript reviewing.

Conflicts of interest

The authors state that there are no conflicts to declare.

Data availability

Experimental details and the data (Fig. S1–S6) that support the findings of this study are available in the supplementary information (SI). See DOI: https://doi.org/10.1039/d5nr03290c.

Acknowledgements

V. B. and C. S. acknowledge a JICA Friendship project. Also, this research was supported by a JSPS Grant-in-Aid for Scientific Research (23H01781). We acknowledge N. Hirai at the Hokkaido University Global Facility centre for HRTEM imaging. A. M. acknowledges a JICA Scholarship for doctoral studies.

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