Faceting-Controlled Zeeman Splitting in Plasmonic TiO2 Nanocrystals

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Faceting-Controlled Zeeman Splitting in Plasmonic TiO Nanocrystals Penghui Yin, Manu Hegde, Natalie S Garnet, Yi Tan, and Pavle V. Radovanovic Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.9b03128 • Publication Date (Web): 26 Aug 2019 Downloaded from pubs.acs.org on August 29, 2019

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Faceting-Controlled Zeeman Splitting in Plasmonic TiO2 Nanocrystals Penghui Yin, Manu Hegde,† Natalie S. Garnet, Yi Tan, and Pavle V. Radovanovic*

Department of Chemistry, University of Waterloo, 200 University Avenue West,

Waterloo, ON N2L 3G1, Canada

KEYWORDS Zeeman splitting, semiconductor plasmonics, nanocrystal morphology, exchange coupling, oxygen vacancy, magnetic circular dichroism

ABSTRACT Dynamic manipulation of discrete states in nanostructured materials is critical for emerging quantum technologies. However, this process often requires a correlation of mutually competing degrees of freedom. Here we report the control of magnetic-field-induced excitonic splitting in colloidal TiO2 nanocrystals by control of their faceting. By changing nanocrystal morphology via reaction conditions, we control the

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concentration and location of oxygen vacancies, which can generate localized surface plasmon resonance and foster the reduction of lattice cations leading to the emergence of individual or exchange-coupled Ti(III) centers with high net-spin states. These species can all couple with the nanocrystal lattice under different conditions resulting in distinctly-patterned excitonic Zeeman splitting and selective control of conduction band states in external magnetic field. This work demonstrates the concept of using nanocrystal morphology to control carrier polarization in individual nanocrystals using both intrinsic and collective electronic properties, representing a unique approach to multifunctionality in reduced dimensions.

Complex metal oxides have a palette of properties which can be chemically controlled, making them attractive for the design of multifunctional nanomaterials with correlated degrees of freedom. Introducing free carriers into colloidal semiconductor metal oxide nanocrystals (NCs) results in their light-stimulated confined resonant oscillation at the interface with the dispersing medium, known as localized surface plasmon resonance (LSPR).1 The ability to control the type and concentration of charge


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carriers in semiconductor NCs allows for broad tunability of LSPR from visible to midinfrared (IR) range, which makes this class of materials attractive for a variety of applications in optoelectronics,2,3 photocatalysis,4,5 and infrared spectroscopy.6 The most common ways to generate and control delocalized charge carriers in semiconductor

NCs

is

through

aliovalent

doping,7-12

photocharging,13,14

and

electrochemical modulation.15,16 High density of “extra” electrons in semiconductor NCs has also been achieved by the formation of charged native defects, such as cation and anion vacancies.4,17-19 The propensity for the formation of oxygen vacancies in many wide band gap metal oxides asserts them as promising candidate materials for vacancy-doped plasmonic semiconductor NCs.4,20-23 Additionally, the formation of oxygen vacancies in TiO2 is accompanied by the formation of Ti3+, which is responsible for the generation of new energy states within the band gap.22 When excited by circularly polarized light of resonance frequency, free electrons in NCs undergo cyclotron motion.24,25 For the opposite light polarization (left circularly polarized (LCP) and right circularly polarized (RCP)) two degenerate cyclotron plasmonic modes having the opposite helicity (ρ- and ρ+, respectively) are formed. In an


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external magnetic field, these two modes split owing to the additional Lorentz force acting on the electrons in cyclotron motion. The difference in absorption of LCP and RCP by these magnetoplasmonic modes results in derivative-shaped magnetic circular dichroism (MCD) signal. The universal behaviors of LSPR MCD signal are linear dependence on the magnetic field and independence on temperature (see Supporting Information for more details). While controlling the LSPR using external magnetic field is by itself an attractive opportunity (e.g., for refractometric sensing and quantitative determination of carrier mobility in colloidal NCs),24,25 the recent discovery of magnetoplasmon-induced carrier polarization in aliovalently-doped In2O3 NCs26 allows for considering plasmonic semiconductor NCs beyond their ability to promote near-field enhancement of optical properties or respond to changes in the local environment. The possibility to impart angular momentum from magnetoplasmonic modes to the conduction band excited states renders degenerate semiconductor NCs truly multifunctional materials with correlated degrees of freedom. The magnetoplasmonic mode can be viewed as an analogue to the electron spin in spintronics,27,28 or discrete valleys of the band structure


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in valleytronics.29 However, magnetoplasmon represents a collective electronic property in contrast to electron spin which is an intrinsic property of an individual electron. This concept

allows

for

instituting

and

controlling

carrier

polarization

directly

in

semiconductor nanostructures at room temperature, opening new possibilities in electronics, optoelectronics, and quantum information technologies. The magnetic-field-mediated plasmon-induced carrier polarization discussed above has so far been demonstrated in aliovalently-doped In2O3 NCs,26,30 as a model system whose electronic structure and plasmonic properties have been thoroughly investigated. Here we use MCD spectroscopy to demonstrate unique excitonic splitting patterns in oxygen-deficient TiO2 NCs enabled by simultaneous control of their faceting and a degree of electron delocalization. The excitonic MCD signal arising from plasmonexciton coupling has the opposite sign for TiO2 NCs relative to aliovalently-doped In2O3 NCs, suggesting the dependence of the plasmon-induced carrier polarization on the NC electronic structure. Furthermore, controlled reduction of Ti(IV) by oxygen vacancies leads to the formation of Ti(III) sites, which can undergo exchange-coupling processes with the host lattice or neighbouring Ti(III) sites. The formation of these Ti(III)-based


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species by different degree of spatial delocalization of vacancy-originated electrons can be controlled by NC morphology, together with the LSPR associated with fully delocalized electrons. The TiO2 NC morphology and faceting, as well as the defect formation in the NCs were controlled by the selection of titanium(IV) halide precursor [TiF4 (F), TiCl4 (Cl), or their mixture (M)] and the coordinating ligand [oleylamine (OLAM) or 1-octadecanol (ODOL)]. We adopted the designation previously used for these types of samples, which specifies the precursor and coordinating ligand using the above abbreviations (FOLAM, M-OLAM, and Cl-ODOL).4 All NC samples exhibit tetragonal anatase crystal structure, as evident from the XRD patterns shown in Figure 1a. The shifting and broadening of (004) and (200) peaks, and the ratio of (105) to (211) peaks have been associated with the degree of truncation of tetragonal bipyramidal anatase NCs perpendicular to direction.4 In case of TiF4 precursor, F⁻ ions can selectively bind to {001} facets of the nucleated anatase TiO2 NCs and act as a morphology-controlling agent,31 allowing F-OLAM TiO2 NCs to adopt strongly truncated bipyramidal morphology with a dominant fraction of {001} facets. Such a high degree of truncation effectively


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gives rise to the formation of nanoplatelets, as shown in the inset of Figure 1a (top panel). On the other hand, M-OLAM and Cl-ODOL samples have reduced {001} faceting and dominant {101} faceting (insets in Figure 1a). The morphology of the NCs from these samples was confirmed by TEM imaging (Figure 1b and Figures S1-S3 in Supporting Information). Additional discussion about the NC formation mechanism and morphology is also given in Supporting Information. As expected based on the XRD peak broadening, which is particularly evident for 2θ ≈ 38° and 55°, the average size of NCs of Cl-ODOL sample is larger than that of M-OLAM sample by a factor of 2 (Figure S4 in Supporting Information). The absorption spectra of TiO2 NC samples in the UV-visible and IR range, normalized to the band gap absorbance (i.e., NC volume), are shown in Figure 1c and d, respectively. In contrast to Cl-ODOL NCs, a strong broad band in the mid-IR range is observed for F-OLAM and M-OLAM NCs (Figure 1c). This band is characteristic for LSPR and gives F-OLAM and M-OLAM colloidal suspensions characteristic blue-green color (inset in Figure 1c). The intensity of LSPR is nearly two times higher for M-OLAM than for F-OLAM sample, indicating a higher density of free conduction band electrons


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in the former. This increase in electron density is accompanied only by a relatively small blue shift of the LSPR band maximum.32 A low sensitivity of plasma frequency (ωp) on carrier density could arise from nonparabolicity of the conduction band in anatase TiO2,33 similarly to degenerately-doped InN.34 It is generally accepted that extra electrons in TiO2 arise from oxygen vacancies, which are responsible for high conductivity of TiO2 bulk samples,35 thin ﬁlms,36 or nanostructures.23 While the redox titration method13,14,34 for counting free electrons in these NCs proved to be challenging, based on Drude model fitting of the LSPR spectra we estimate that the free electron densities of F-OLAM and M-OLAM NCs are on the order of 1020 cm-3.37 The concentration of localized electrons is expected to be a fraction of this value (see below). The increase in the LSPR intensity is accompanied by a blue shift in the band edge absorption (Figure 1d) due to increased filling of the conduction band states (Burstein-Moss shift). It is relevant to note that the band edge absorption of M-OLAM lies at a higher energy relative to that of F-OLAM sample, in spite of F-OLAM NCs being significantly smaller in one dimension (truncated bipyramid height), confirming that the blue shift is not due to quantum confinement. Given the absence of LSPR in Cl-ODOL


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NCs, this sample will serve as a control experiment in the magneto-optical measurements (see below).

Figure 1. (a) XRD patterns of F-OLAM, M-OLAM and Cl-ODOL TiO2 NCs described in the text. The corresponding NC shapes are shown as insets. (b) Overview (left) and lattice-resolved (right) TEM images of F-OLAM (top panels), M-OLAM (middle panels) and Cl-ODOL (bottom panels) NC samples in (a). The relevant lattice planes are indicated in the lattice-resolved images in the right hand side panels. (c,d) Optical absorption spectra of the same NC samples in the mid-IR range (c) and UV-visible range (d), showing LSPR and band gap absorption, respectively. The absorption


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spectra are normalized to the NC volume. The photographs of the colloidal suspensions of nanocrystals are shown in the inset in (c).

To investigate the local electronic structure of TiO2 NCs we performed comparative Xray photoelectron spectroscopy (XPS) and X-ray absorption near-edge spectroscopy (XANES) investigations. Figure 2a shows Ti 2p XPS spectra of M-OLAM (bottom), FOLAM (middle), and Cl-ODOL (top) TiO2 NCs. The two peaks centered at 458.5 eV and 464.3 eV in the spectra of Cl-ODOL and F-OLAM samples are readily assigned to the 2p3/2 and 2p1/2 doublet transition, respectively, characteristic for Ti4+. However, in contrast to the spectra of F-OLAM and Cl-ODOL, M-OLAM spectrum clearly shows asymmetric structure and broadening at the low energy side of both spectral bands. Multiple peak fitting reveals the presence of additional peaks at 457.7 eV and 462.6 eV, which are characteristic for Ti3+ 2p3/2 and 2p1/2, respectively. We further investigated the coordination number and local symmetry of Ti sites using XANES (Figure S5). The preedge features of XANES spectra are particularly sensitive to the Ti coordination environment, because the local symmetry affects the degree of p-d orbital mixing.38 The


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pre-edge spectrum of Cl-ODOL (Figure 2b, top panel) shows four characteristic peaks with relative intensities similar to the spectra reported for anatase TiO2.39,40 On the basis of an empirical correlation between the A2/A3 peak ratio and the particle size, the increased A2 peak intensity has been attributed to the increased contribution from fivecoordinate Ti centers mostly residing in the surface vicinity.39 Similar trend is observed in Figure 2b. The A2/A3 peak ratio for F-OLAM (middle panel) and M-OLAM (bottom panel) increases noticeably compared to that for Cl-ODOL sample, consistent with their smaller sizes and larger surface-to-volume ratios. Importantly, although F-OLAM NCs have larger surface area to volume ratio than M-OLAM NCs, owing to their platelet-like morphology, the A2/A3 peak ratios are similar. This observation suggests that the additional Ti sites with a lower coordination number reside in the interior of M-OLAM NCs, which can be associated with internal oxygen vacancies.


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Figure 2. (a) Ti 2p XPS spectra of Cl-ODOL (top panel), F-OLAM (middle panel) and MOLAM (bottom panel) TiO2 NC samples. Best fits to the experimental data (black squares) are shown with red lines and the shaded areas represent deconvoluted Ti4+ (blue) and Ti3+ (orange) 2p doublets. (b) Pre-edge XANES spectra of Cl-ODOL (top panel), F-OLAM (middle panel) and M-OLAM (bottom panel) samples. Best fits to the experimental data (black squares) are shown with red lines and the shaded areas represent deconvoluted characteristic peaks indicated in the top panel. Light blue line is the fit to the leading edge of the shoulder feature in the XANES spectrum located at ca. 4980 eV.

Figure 3 (lower panel) compares the MCD spectra of F-OLAM and M-OLAM samples in the UV-visible range. The dominant feature in both spectra is the positive band that


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coincides with the corresponding band edge absorption (upper panel of Figure 3). In contrast to F-OLAM and M-OLAM, no MCD signal is observed for non-plasmonic ClODOL TiO2 NCs (dashed trace in Figure 3). This comparison confirms that the LSPR is necessary for inducing band splitting in TiO2 NCs using magnetic field and circularly polarized light. Unlike LSPR MCD signal, which originates from cyclotron motion of free electrons,24,25 the excitonic MCD intensity involves Zeeman splitting of the excited electronic states. LSPR can be indirectly excited by circularly polarized light via coupling with the exciton, while the resulting magnetoplasmonic modes induced by an external magnetic field impart angular momentum to the excited states around the Fermi level.26 The mechanism by which cyclotron magnetoplasmonic modes are generated and coupled with exciton to produce aforementioned Zeeman splitting is a subject of an ongoing investigation. However, an important component of this mechanism could be lattice phonons, because they satisfy key requirements to mediate the process; phonons can couple with both excitons41 and plasmons42 in semiconductors, and simultaneously transfer angular momentum,43 allowing for the formation of the magnetoplasmonic modes and splitting of the conduction band states in a magnetic


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field. Importantly, the sign of the excitonic MCD band is opposite from that observed for plasmonic In2O3 NCs (Figure S6),26,30 indicating reversed splitting pattern of the excited states. The ramification of these results is that plasmon-induced carrier polarization can be controlled by the electronic structure of the NC host lattice. While these considerations require further investigations and are beyond the scope of this work, one of the notable differences between In2O3 and TiO2 is that the nature of the conduction band states.7,44 To examine the electronic states of plasmonic TiO2 NCs we performed density functional theory (DFT) calculations of ideal and vacancy-containing TiO2 (Figure S7). Unlike the conduction band of In2O3, which has In s-orbital character, the conduction band of anatase TiO2 consists predominantly of symmetry-sensitive Ti dstates (Figure S7).44 Specifically, the lower part of the TiO2 conduction band is dominated by dxy orbitals (Figure S7d). This hypothesis was confirmed for WO2.72 NCs (Figure S8), which also have the conduction band states made up of W 5d orbitals.45


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Figure 3. Optical absorption (top panel) and MCD (bottom panel) spectra of F-OLAM (blue trace) and M-OLAM (green trace) NCs in the UV-visible region. MCD spectrum of Cl-ODOL (black dashed trace) is shown for comparison.

Although the excitonic splitting is common for both M-OLAM and F-OLAM TiO2 NCs, there are some notable differences between the MCD spectra of these two samples. The spectrum of M-OLAM has a set of low-intensity spectral features around 1.8 eV (see also Figure S9), and a broad structureless band having a negative sign and a maximum intensity at ca. 3.1 eV (400 nm). We assign the low-energy spectral features from 1.6 to 2.1 eV to symmetry-split 2T22E transition, characteristic for six-coordinate


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Ti3+, based on their energy and intensity.46 The energy range of the band centered at 3.1 eV appears to be too high for intra-ionic (d-d) ligand-field transitions. Instead, in d1 complexes, such as Ti(III) and V(IV), such strong broad bands lying above ~ 2.5 eV are generally assigned to interionic (charge transfer) transitions (see below).47,48 Another difference between M-OLAM and F-OLAM MCD spectra, which is particularly important for this work, is related to high-temperature excitonic MCD intensity (Figure S10). The signal intensity at 300 K is stronger for M-OLAM sample, and concurs with the trend of LSPR intensity in Figure 1c. This result is consistent with the conclusion that MCD spectra of the excitonic transitions gain intensity through coupling with the magnetoplasmonic modes.30 The MCD spectra of F-OLAM TiO2 NCs collected at 5 K and different magnetic fields from 1 to 7 T are shown in Figure 4a. The integrated excitonic MCD intensity follows ideal linear dependence on the magnetic field (Figure 4b), reflecting the magneto-optical behavior of LSPR. The intensity of this MCD signal decreases with temperature from 5300 K by ca. 20 % (Figure 4c). The excitonic MCD intensity decay with temperature can be fit to Curie-Weiss law:


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

C T  TC

(1)

where χ is magnetic susceptibility, TC is the Curie temperature, T is temperature ranging from 5 to 300 K, and C is the material-specific Curie constant. The agreement of the fit with the experimental data, as shown in Figure 4d, suggests that the decrease in the signal intensity with temperature is associated with paramagnetic Ti3+ centers formed by reducing a small fraction of Ti4+ host lattice cations.49 Importantly, these results suggest that excitonic splitting in self-doped TiO2 NCs can be attained without the need for an external magnetic impurity.50 The component of the excitonic MCD signal that remains constant at high temperatures represents the contribution from the magnetoplasmoninduced carrier polarization, which is independent of temperature.


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Figure 4. (a) 5 K MCD spectra of F-OLAM TiO2 NCs in the band gap region collected for different magnetic field strengths (1-7 T). (b) Magnetic field dependence of integrated MCD intensities determined from the spectra in (a). (c) 7 T MCD spectra of F-OLAM TiO2 NCs in the band gap region collected for different temperatures (5-300 K). (d) Temperature dependence of the integrated MCD intensities determined from the spectra in (c). (e) Schematic representation of the excited state splitting in TiO2 NCs induced by angular momentum of the cyclotron magnetoplasmonic modes (dashed red and blue lines). The splitting causes the difference in absorption of LCP (vertical blue arrow) and RCP (vertical red arrow) light with selection rule ΔMJ= ± 1.


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The field and temperature dependences are more complex for M-OLAM samples (Figure 5). The MCD spectra of this sample for different magnetic fields are shown in Figure 5a. The intensities of symmetry-split ligand-field transitions at ~ 1.8 eV saturate with increasing magnetic field strength (Figure S9a), although they are too weak to accurately integrate. Even though the negative band at ca. 3.1 eV and the excitonic transition overlap to some degree, their deconvolution through peak fitting allows for extraction of the integrated intensities (Figure S11). The field dependencies of these integrated intensities are compared in Figure 5b. Surprisingly, the Brillouin function (equation 2) fits best to the intensities of the negative band (red diamonds in Figure 5b) for the net spin state S=1 (rather than S=1/2 which would correspond to isolated noninteracting Ti3+ sites) and the Landé g-factor of 1.97 characteristic for six-coordinate Ti3+.51,52

   g  B  g  B 1 M S  Ng S B (2S  1) coth  (2S  1) S B   coth  S B   2 2kBT    2kBT   

(2)

In equation 2, MS is the sample magnetization, B is the magnetic field strength, T is the temperature, and N, B , and kB are the number density of magnetic centers, the


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Bohr magneton, and the Boltzmann constant, respectively. This analysis further suggests that this band is not due to a ligand-field (d-d) transition, but rather charge transfer between a neighboring oxygen ion (as a ligand) and Ti3+ ion (as a metal) involved in a dinuclear complex consisting of two ferromagnetically-coupled Ti3+ centers. From the solid-state band structure perspective, ligand can also be viewed as a part of the valence band given its O 2p character (Figure S7). This conclusion is consistent with the formation of a triplet ground state (S=1) involving exchange coupling of two Ti3+ ions, enabled by a bridging oxygen vacancy (Figure 5e).53 It has been proposed that two extra electrons trapped by initially ionized oxygen vacancy site ( VO•• ) are localized on two individual nearest-neighbor cations, forming a pair of Ti3+ ions which are exchange-coupled to give the net spin state of 1.54 Even higher net spin states have been reported in some reduced anatase TiO2 NCs, with four ferromagnetically coupled Ti3+ ions having an effective spin state S=2.55 Based on the comparison with the absorption spectra of six-coordinate Ti3+ complexes and the observed magnetic field dependence, we assign the negative MCD band in Figure 5a to


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Ti3+-O ligand-to-metal change transfer transition involving exchange-coupled Ti3+ (Figure 5e). We were particularly interested in the magnetic field and temperature dependences of the excitonic MCD spectrum of M-OLAM relative to F-OLAM sample. Although the MCD dependence on the magnetic field for the excitonic transition of M-OLAM (Figure 5b, blue triangles) differs from that for the charge transfer band (Figure 5b, red diamonds), it also significantly deviates from linearity expected for magnetoplasmon-induced band splitting. The difference between the temperature dependences of these two transitions is even more pronounced (Figs. 5c and d). The charge transfer band decreases more strongly with temperature, and becomes undetectable at 300 K. On the other hand, excitonic band retains ca. 60 % of its 5 K intensity at room temperature. The decrease in both signals follows the Curie-Weiss law (equation 1), consistent with spin-induced Zeeman splitting (solid lines in Figure 5d). These results suggest that excitonic splitting consists of two contributions, similarly to F-OLAM samples ─ magnetoplasmon-induced splitting which is temperature independent, and anomalous Zeeman splitting which decreases with temperature. However, the best fit for the field dependence of the


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excitonic MCD intensity is obtained for a combination of the linear fit and the Brillouin function for S=1 (solid blue line in Figure 5b and Figure S12). These results demonstrate not only that the oxygen vacancies serve a dual purpose (to mediate the formation of high-spin state complexes involving Ti3+ and to generate LSPR), but that the newly formed high-spin complexes can act as highly-selective spin polarizers in TiO2 NCs. To our knowledge, this is the first demonstration of a complex consisting of exchange-coupled impurity sites serving as an artificial quasi-molecular dopant that selectively induces splitting of the semiconductor band states. This result opens the door for using redox chemistry involving native defects to engineer and control quantum states.


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Figure 5. (a) 5 K MCD spectra of M-OLAM TiO2 NCs collected for different magnetic field strengths (1-7 T). (b) Magnetic field dependence of the integrated MCD intensities determined from the spectra in (a) for different transitions, as indicated with the corresponding symbols. Red line is the best fit of the integrated intensity of the negative band centered at ca. 3.1 eV with the Brillouin function (equation 2) for S=1. Blue line is the best fit of the integrated excitonic intensity with a combination of the linear dependence and Brillouin function for S=1. Dashed blue line designates ideal linear dependence characteristic for magnetoplasmon-induced excited state splitting. (c) 7 T MCD spectra of M-OLAM TiO2 NCs collected for different temperatures (5-300 K). (d)


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Temperature dependence of the integrated MCD intensities determined from the spectra in (c) for different transitions, as indicated with the corresponding symbols. The lines are best fits of the Curie-Weiss law (equation 1) to the corresponding experimental data points. (e) Schematic representation of a complex involving two neighboring Ti ions (left) and their reduction and coupling induced by oxygen vacancy (right). Titanium sites are shown with light blue spheres, oxygen sites with red spheres, and oxygen vacancy with open dotted circle. The two neighbouring titanium ions reduced by the oxygen vacancy are exchange-coupled giving the net spin state S=1. Charge transfer transition involving exchange-coupled Ti3+ are indicated with the arrow.

The origin of Ti3+ formation and the correlation between oxygen vacancies and Ti3+ in different TiO2 NC samples were further explored by EPR spectroscopy (Figure S13a). In the case of F-OLAM sample, a relatively strong signal at g≈1.93 and a weaker narrower signal at g≈2.00 were observed. The signal at g=2.00 can be associated with oxygen radical species (e.g., O- or O2-) on TiO2 surfaces,56

while that at g≈1.93 to

paramagnetic Ti3+ ions located on or near the surface of the NCs.22,56 These EPR


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features are also observed for M-OLAM sample, although they appear less pronounced owing to the reduced fraction of {001} facets relative to F-OLAM NCs. In addition, the EPR spectrum of M-OLAM sample shows a strong broad signal at g≈1.97. This signal, which has been attributed to the presence of internally-incorporated Ti3+, is frequently associated with the formation of bulk oxygen vacancies (see also Figure S7 and the associated discussion in Supporting Information),52 and its broadening is indicative of Ti3+ interactions. Internal vacancy-enabled self-doping of TiO2 NCs with Ti3+ results in the formation of Ti3+ exchange-coupled pairs which generate high net spin state and contribute to the NC band splitting. To gain further insight into the mechanism of band splitting in TiO2 NCs, we compared the magneto-optical results with those of the magnetization measurements. The saturation magnetization for M-OLAM sample is ca. 60 times larger than that for F-OLAM, suggesting a higher concentration of paramagnetic centers (Figure S13b). The saturation magnetization for F-OLAM NCs is best fit with S=1/2 Brillouin function, assuming Landé g-factor characteristic for surfacebound Ti3+ (g=1.93). A small deviation from the ideal spin-only saturation behavior is likely related to site symmetry-induced spin-orbit coupling (Figure S13b, inset). The best


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fit to the magnetization data for M-OLAM sample, on the other hand, is achieved for S=1 spin state which corresponds to Ti3+ exchange-coupled pairs, as discussed above (see also Figure S14). From these and the results in Figure 1c, it is evident that both free electron concentration (responsible for LSPR intensity) and localized Ti3+ centers (responsible for saturation magnetization) increase in parallel, enabled by the formation of the oxygen vacancy defects. The main difference between Ti3+-induced band splitting in F-OLAM and M-OLAM samples is related to the NC dimensionality.

The

concentration of internal oxygen vacancies and the resulting Ti3+-VO-Ti3+ complexes, which are dominant in M-OLAM NC sample, are negligible in F-OLAM nanoplatelets because of the small fraction of bulk. The Ti3+-related band splitting in F-OLAM can, therefore, be associated only with non-coupled Ti3+ in the vicinity of the NC surfaces. In summary, we demonstrated highly selective morphology-induced control of carrier polarization in oxygen-deficient TiO2 NCs. Simultaneous generation of LSPR and selfdoping in TiO2 NCs arise from the intrinsic redox chemistry involving oxygen vacancies, which can be controlled via surface ligation and NC morphology. The fluoride-induced oxygen vacancy electronic states are responsible for donating free electrons to the


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conduction band and the formation of cyclotron magnetoplasmonic modes upon excitation with circularly polarized light in a magnetic field. Free electrons arising from the presence of oxygen vacancies can also localize on titanium sites, reducing Ti(IV) to Ti(III) which acts as a self-dopant. In F-OLAM TiO2 nanoplatelets having a large fraction of fluoride-stabilized {001} facets, oxygen vacancies are predominantly formed as nearsurface defects, resulting in localized surface-segregated Ti(III) sites with single unpaired electrons (S=1/2). On the other hand, in truncated square bipyramidal MOLAM NCs having a smaller percentage of {001} surface area and larger bulk volume, oxygen vacancies can be stabilized internally, resulting in the formation of two adjacent vacancy-bridged Ti(III) sites which are exchange-coupled to produce triplet state (S=1). Surprisingly, all species (magnetoplasmon, individual and exchange-coupled Ti(III) centers) lead to distinct excitonic splitting pattern. Controlling the redox properties of specific vacancy sites allows for selective carrier polarization and the formation of discrete quantum states. The main ramification is this work is that complex multifunctional materials with multiple interacting degrees of freedom (plasmon, spin, and charge), relevant for emerging quantum technology applications, can be designed


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by simple control of the morphology and faceting of colloidal plasmonic semiconductor NCs.

ASSOCIATED CONTENT

Supporting Information. Experimental and theoretical methods, additional TEM images of TiO2 NCs, dynamic light scattering data for TiO2 NCs, XANES and MCD spectra and analysis of TiO2 NCs, TiO2 band structure calculations, characterization of WO2.72 NCs, magnetic susceptibility data and fitting for TiO2 NCs, EPR spectra of TiO2 NCs, additional discussion about the origin of magnetoplasmonic modes, TiO2 NC faceting, and DFT results (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]

Present Addresses


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†Department

of Physics, Simon Fraser University, 8888 University Drive, Burnaby, BC,

V5A 1S6, Canada

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENT

This research was undertaken thanks in part to funding from the Canada First Research Excellence Fund. We acknowledge the financial support by the Natural Sciences and Engineering Research Council of Canada (RGPIN-2015-04032 and RTI-2016-00012), and Canada Foundation for Innovation (Project # 204782). This research used resources of the Advanced Photon Source (APS), a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. We thank Dr. Zou Finfrock and Dr. Debora Motta Meira for their assistance at the 20-BM beamline at APS. The electron microscopy research described in this paper was performed at the Canadian Centre for Electron Microscopy at McMaster University, which is supported by


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NSERC and other government agencies. P.Y. thanks Waterloo Institute for Nanotechnology (WIN) for a Graduate Research Fellowship and Canadian Light Source for a Graduate Student Travel Award (CLS@APS).

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Faceting-Controlled Zeeman Splitting in Plasmonic TiO2 Nanocrystals

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