Enhancement of X-ray-Excited Red Luminescence
of Chromium-Doped Zinc Gallate via Ultrasmall Silicon Carbide Nanocrystals
X-ray-activated near-infrared luminescent nanoparticles are considered as new alternative optical probes due to being free of autofluorescence, while both their excitation and emission possess a high penetration efficacy in vivo. Herein, we report silicon carbide quantum dot sensitization of trivalent chromium-doped zinc gallate nanoparticles with enhanced near-infrared emission upon X-ray and UV–vis light excitation. We have found that a ZnGa2O4 shell is formed around the SiC nanoparticles during seeded hydrothermal growth, and SiC increases the emission efficiency up to 1 order of magnitude due to band alignment that channels the excited electrons to the chromium ion.
Introduction<!>General Physical Properties
of NPs<!><!>General Physical Properties
of NPs<!><!>General Physical Properties
of NPs<!>Optical Properties<!><!>Optical Properties<!>Local Environment of Trivalent
Chromium<!><!>Local Environment of Trivalent
Chromium<!>Variation of Reaction Parameters<!><!>Variation of Reaction Parameters<!>Studies on Reaction Kinetics<!>Energy Levels in ZGO/Cr–SiC<!><!>Energy Levels in ZGO/Cr–SiC<!>Discussion<!>Conclusions<!>Materials<!>Synthesis Procedures<!>Sample Preparation for
PL and XEOL Measurements<!><!>Author Contributions<!><!>Notes
<p>Zinc
gallate (ZnGa2O4 or ZGO) has attracted
increased attention for a broad range of optical applications owing
to its excellent thermal and chemical stability and a wide band gap
(≈4.4–5.2 eV).1−3 It is known to act as a phosphor
host with one example being chromium doping of the ZGO lattice (henceforth
denoted ZGO/Cr). ZGO/Cr displays a red–infrared luminescence
at around 700 nm under a wide range of excitations4,5 when
the Cr3+ ions replace Ga3+ in the crystal lattice.
As a consequence, ZGO/Cr is one of the leading candidates for realizing
optical imaging of vascularization, tumors, and grafted cells.6,7</p><p>The emission spectrum of the Cr3+ ion is closely
related
to its specific atomic structure, exhibiting three electrons in the
highest energy d-orbital (i.e., 3d3).
Inside the spinel ZGO structure (representing a C3v crystal
field), the degenerate 3d orbital of Cr3+ splits into three
sublevels (2eg and t2g). The 3d3 multiplet
states are 4A2, 2E, 4T2, 4T1, and 4T1 (3P). The main radiative transition occurs between the 2E–4A2 states and is responsible for
the near-infrared emission. An unperturbed Cr3+ ion has
two sharp photoluminescence (PL) lines without participation of vibrations
called zero phonon lines (ZPLs), namely, R1 and R2, at 688.0 and 688.8
nm at room temperature (RT), respectively. Due to trigonal distortion
in the ZGO crystal, the Cr3+ ion exhibits additional ZPLs,
often marked as N1, N2, and N3 lines.4 The
N3 line is located at around 700 nm and is attributed to Cr3+–Cr3+ pairs.8,9 Very close to N3, another
peak can emerge, labeled n7, which originates from more complex Cr
clusters. The N2 line is at 695 nm and is unambiguously connected
to a first neighbor cationic anti-site defect and an inverse spinel
structure around the Cr3+ ion.8,10 The
origin of N1, located at 690 nm, is more controversial.11,12 Such defects due to trigonal distortion alter the optical properties
by shifting the peak maxima and affect the emission intensity and
the exciton-relaxation time.</p><p>The multiple excited states of
Cr3+ allow exciton relaxation
from the conduction band (CB) of the host material (i.e., ZGO) to the lowest excited state of Cr3+. The perturbed
local environment of the dopant can create trap states, that is, can
store electrons and channel them to the dopant excited state at a
later stage, creating a strong and long-lasting luminescence. This
mechanism, together with the high X-ray absorption cross-section of
the host, allows for high-energy excitation and makes the material
suitable as a phosphor and scintillator material. Indeed, optical
imaging could be advantageously carried out by using a phosphor as
a luminescent probe, emitting in the red–infrared part of the
spectrum upon X-ray excitation. In fact, in some cases, emission without
excitation has been reported in the literature.13 A red–infrared emission is necessary for luminescence
to pass through human tissues.14 By using
persistent luminescent nanoparticles (NPs) and/or the capability of
X-ray excitation to penetrate the body, the autofluorescence of tissue
can be avoided.3,13,15 The use of such luminescent systems for in vivo imaging is of great interest to investigate pathologies in animal
models and visualize deep-tissue cancer cells.</p><p>A couple of attempts
were carried out to improve the emission intensity
of ZGO, either by varying the Zn ratio,5 using Bi3+ doping,16 or mixing
in conducting oxides such as In2O3.17 However, such methods either require precise
stoichiometric control of multiple components, as the desired luminescence
is very sensitive to dopant concentrations, or multiphase systems,
respectively.</p><p>The most accepted method to synthesize ZGO/ZGO/Cr
NPs (NPs) is
the transformation of the oxides or hydroxides of the elements into
crystalline ZGO. This can be accomplished by annealing an oxide or
hydroxide mixture at high temperature or by applying solvothermal
methods at a moderate temperature.18−21 The benefit of solvothermal,
in particular hydrothermal, synthesis over the solid–state
reaction is the significantly lower reaction temperature and a more
controllable particle size.21 The latter
is of importance for in vivo applications as the
particle size has a huge impact on the cellular uptake, circulation
time, and toxicity. However, the reaction mechanism of the hydrothermal
synthesis of ZGO/ZGO/Cr has not been fully understood to date.</p><p>Here, we report on a considerable improvement of the synthesis
method and optical properties of the ZGO/Cr system using silicon carbide
(SiC) ultrasmall NPs22,23 as both seeds and optical sensitizers.
The emission intensity of ZGO/Cr with a SiC core (i.e., ZGO/Cr–SiC) is found to be an order of magnitude higher
than that of ZGO/Cr under X-ray and UV–vis light excitation
with a wavelength of 250 nm, whereas it is 2 times higher under 290
nm UV–vis light excitation. Such an enhancement is significantly
larger than that observed in previous reports16,24−26 while, to the best of our knowledge, excitation wavelength-dependent
enhancement has not been reported to date. Reaction kinetic studies
suggest that ultrasmall SiC NPs can reduce the formation energy barrier
of ZGO/Cr during hydrothermal synthesis allowing for faster ZGO/Cr
particle growth at the early reaction stage and channels the excitons
to the Cr3+ ions in the lattice, thus improving the emission
intensity significantly under high-energy excitation. Such a seeding
effect is found to be size-selective, and only ultrasmall SiC NPs
with a diameter below 3 nm were found to participate in the colloid
reaction.</p><!><p>The comparison of the structure, crystallinity, size,
and morphology
of ZGO/Cr and ZGO/Cr–SiC results in many similar features (Figure 1). The broad reflections
of the X-ray diffraction (XRD) pattern in both cases (Figure 1a) correspond to the cubic spinel crystal structure of ZnGa2O4 [Crystallography Open Database (COD) ID 4001767,
space group , a = 8.35 Å].
The
Rietveld analysis of the profile indicated 10 and 9.5 nm crystallite
size for the ZGO/Cr and ZGO/Cr–SiC samples, respectively. The
Rietveld refinement of the properly restricted tetrahedral and octahedral
site occupancies for Zn2+, Ga3+, and Cr3+ yielded a ∼10% degree of inversion of the spinel
structure (continuous parametrized transition from the spinel to the
inverse spinel structure). Reflections from the SiC seeds, which should
appear as 3 times more broadened reflections with respect to ZGO,
are not visible in the pattern due to the low volume fraction of SiC
(2.7%), evidencing that SiC seeding does not perturb the XRD pattern
of ZGO/Cr. The lattice spacing, measured via high-resolution
transmission electron microscopy (HR-TEM) analysis (Figure 1b,c) and selected area electron
diffraction patterns (EDS) patterns (not shown), also confirms that
the ZnGa2O4 structure was synthesized.</p><!><p>(a) XRD of
ZGO/Cr and ZGO/Cr–SiC NPs with reference to the
ZnGa2O4 crystal structure (COD). (b) HR-TEM
images of ZGO/Cr and (c) ZGO/Cr–SiC NPs. The insets in (b)
and (c) show the size distribution calculated from several HR-TEM
images.</p><!><p>Scanning electron microscopy (SEM),
EDS, and TEM–EDS confirm
the presence of Zn, Ga, O, Si, and C in the ZGO/Cr–SiC NPs
and Zn, Ga, and O in the ZGO/Cr NPs (Table 1). The size-distribution measurements from
HR-TEM analysis gave very similar results to XRD, namely, 9 nm for
ZGO/Cr and 9.5 nm for ZGO/Cr–SiC (insets, as shown in Figure 1b,c). The hydrodynamic
sizes measured by means of dynamic light scattering (DLS) in aqueous
solution were 28 and 29 nm with a dispersity of 3.2 and 3.4 for ZGO/Cr
and ZGO/Cr–SiC NPs, respectively. The significant difference
between the sizes measured by XRD/HR-TEM and DLS can be explained
by either cluster formation/aggregation as a result of the enthalpy
minimization by decreasing the surface free energy. This leads to
the presence of large agglomerates which hamper the detection of smaller
individual particles by DLS19 or interparticle
interaction that reduces the average diffusion speed of the particles
causing an overestimation in the particle size.27 Nevertheless, the DLS data correlate well with the other
parameters measured in reaction kinetics experiments implying reasonable
accuracy.</p><p>The HR-TEM image, as shown in Figure 1c, reveals the core–shell
structure
of ZGO/Cr–SiC NPs since Si and C are lighter elements than
Zn and Ga, causing a brighter NP center in the TEM images. There was
no sign of free SiC NPs and/or SiC NPs aggregated onto the surface
of the ZGO NPs. Based on the HR-TEM images, we have calculated that
80% of the particles have core–shell structures. Additional
TEM images and extended discussion can be found in the Supporting Information.</p><p>X-ray photoemission
spectroscopy (XPS) chemical analysis of ZGO/Cr
and ZGO/Cr–SiC is reported in Figure 2, where fitted O 1s and Ga 3p + Si 2p core
level spectra are shown. The spectral features of the O 1s peak, as
shown in Figure 2a,
are similar for both samples and correspond to O2– species (green component) and oxygen vacancies (O-vac; blue component)
of ZGO located at binding energies (BEs) of 529.6 and 530.3 eV, respectively,
and surface hydroxide groups (red component) at BE = 531.3 eV. As
shown in Figure 2b,
the Ga 3p peak (red component) is observed at 104.3 eV (104.5 eV)
for ZGO/Cr (ZGO/Cr–SiC). Different to ZGO/Cr, the ZGO/Cr–SiC
sample exhibited an additional peak (green component) at 101.4 eV,
which is attributed to the Si 2p core level of SiC/SiOC.</p><!><p>Fitted XPS
(a) O 1s and (b) Ga 3p–Si 2p core level spectra
of ZGO/Cr and ZGO/Cr–SiC NPs.</p><!><p>Despite the similar crystal structure and particle geometry, ZGO/Cr–SiC
NPs exhibit unambiguously brighter luminescence upon the same conditions
(see results below). In order to understand such differences, we studied
the local environment of the Cr3+ ion, as well as the reaction
kinetics, along with the excitation and relaxation paths in both systems.</p><!><p>The optical properties
of ZGO and ZGO–SiC NPs were carefully studied with and without
Cr3+ ion doping. Without Cr3+ ions, the bare
ZGO NPs exhibit a weak broad PL peak centered at around 400 nm (see Figure 3a), whereas ZGO–SiC
NPs show enhanced emission properties with an emission maximum at
435 nm. It is known that ZGO usually exhibits a blue–green
emission, due to a self-activation center,28 and SiC NPs yield luminescence at 435 nm when carboxylic surface
groups are coordinated with alkali-metal ions.29 Even though the two aforementioned PL spectra overlap,
the PL excitation (PLE) spectrum of ZGO–SiC NPs shows an additional
excitation peak at around 320 nm (Figure 3b), where SiC NPs have a maximum, suggesting
a SiC-sensitizing effect in a Cr-free ZGO structure. However, these
broad emission lines disappear when Cr is present during the reaction
(see Figure 3c,d).</p><!><p>(a) RT-PL
(290 nm UV illumination) and (b) PLE spectra of ZGO and
ZGO–SiC NPs without Cr3+ ion doping. The asterisk
in (a) marks the Raman peak of water, whereas the black curve in (a)
represents the ZGO spectrum multiplied by 15 to compare luminescence
shapes and maxima. The inset in (b) shows the PLE peak of ZGO–SiC
originating from the SiC NPs. (c) RT-PL (290 nm UV illumination) and
(d) XEOL spectra (non-monochromatized X-rays, 5–30 keV) of
ZGO/Cr and ZGO/Cr–SiC NPs. The inset in (b) shows the PLE peak
of ZGO–SiC originating from the SiC NPs. Insets (c,d) show
spectra normalized to peak intensity.</p><!><p>In the presence of Cr3+, both room-temperature PL (RT-PL;
upon 290 nm UV illumination, see Figure 3c) and XEOL spectra (upon non-monochromatized
X-ray radiation, see Figure 3d) show the typical emission by Cr3+ ions in a
ZGO structure at a wavelength around 700 nm, with the absence of other
emissions for ZGO/Cr and ZGO/Cr–SiC NPs. In addition to the
main ZPLs, Stokes and anti-Stokes phonon sidebands (PSBs) appear as
low- and high-energy shoulders in the PL spectra, broadening the entire
emission spectra.</p><p>Under 290 nm UV illumination (Figure 3c), ZGO/Cr–SiC NPs show
a two-fold
higher PL emission intensity than ZGO/Cr NPs, in both colloid solution
and powder form. Interestingly, the XEOL emission intensity of ZGO/Cr–SiC
NPs under hard X-ray radiation (i.e., non-monochromatized
5–30 keV X-ray light in Figure 3d and monochromatized high-flux 21 keV X-ray light
in the Supporting Information) and 250
nm UV illumination (see the Supporting Information) is more than an order of magnitude higher than the emission from
ZGO/Cr NPs. Such an enhancement is significantly larger than previously
reported,16,24−26 while, to the best of
our knowledge, excitation wavelength-dependent enhancement has not
been reported to date.</p><!><p>It is commonly known that the local environment
of an emission
center alters its optical properties. The absence of the SiC emission
peak in ZGO/Cr–SiC NPs implies a strong interaction between
the NPs' SiC core and Cr3+ ions. Therefore, the
different local environment around Cr3+ appears in ZGO/Cr
and ZGO/Cr–SiC NPs. The fine structure of the observed PL,
as well as electron spin resonance (ESR) spectroscopy, can probe the
local environment.4,8,12,13,30 Under UV illumination
at RT (Figure 3c),
the only difference between ZGO/Cr and ZGO/Cr–SiC is a small
increase in the peak broadening due to PSBs (Stokes) in ZGO/Cr NPs.
Under X-ray excitation (Figure 3d), the emission maximum is slightly red-shifted. The shift
is more intense in ZGO/Cr–SiC NPs although the difference between
the samples is as small as 2 nm. Nevertheless, ZGO/Cr and ZGO/Cr–SiC
NPs show similar transitions. At a low temperature of 4 K [see the
low-temperature PL (LT-PL) spectra in Figure 4a,b], the ZPLs (i.e., R1,
R2, N1–N3, and n7) are mostly resolved, due to decreased thermal
broadening. At this temperature, the N2 line has the highest intensity
in either sample. By comparing the LT-PL spectra of the two samples,
it can be seen that the N1 line is visible in ZGO/Cr but not resolved
in ZGO/Cr–SiC. Furthermore, the N3 (and/or n7) line (corresponding
to Cr3+ cluster defects) has a significantly higher contribution
in ZGO/Cr–SiC NPs than that in ZGO/Cr NPs, and the whole spectrum
is broader due to the higher contribution of the PSBs. In addition
to the sharp PL lines, the ZGO/Cr–SiC sample exhibits two unknown
weak, but broad, peaks at 880 and 935 nm.</p><!><p>(a) LT-PL spectra of
ZGO/Cr and ZGO/Cr–SiC NPs. (b) LT-PL
around 700 nm, showing the Cr peak with the corresponding ZPLs. (c)
ESR spectra of ZGO/Cr and ZGO/Cr–SiC NPs, and the components
for fitting from ref (4). The corresponding PL lines are indicated in parentheses.</p><!><p>The ESR measurements (Figure 4c) were used to correlate the centers, as
found in
the LT-PL spectra (Figure 4a,b). Gourier et al.(4) extensively studied the origin of the red luminescence of Cr3+ ions in ZGO using mostly Q-band ESR spectroscopy. They identified
six types of Cr3+ ions exhibiting different neighboring
defects. We use the reported parameter set as a starting point to
simulate our X-band ESR spectra of the ZGO/Cr and ZGO/Cr–SiC
NPs. Similar to the LT-PL spectra, the ESR spectra of ZGO/Cr are found
to be more complex than ZGO/Cr–SiC and need to be reconstructed
using five different parameters for each Cr center. In contrast, the
ZGO/Cr–SiC spectra can be reconstructed with three parameters,
with a high contribution from the so-called CrÎ³ and
φ centers. Gourier et al.(4) associated the Crγ ESR center with the
PL signal labeled as the N2 line and the φ ESR center with the
n7 line.</p><p>Vibrational spectroscopy such as Raman and FTIR can
reveal local
differences in the crystal structure and was used as a probe of the
local environment of the Cr3+ ions from the crystal side.
Our Raman and FTIR data show higher inverse spinel concentration in
ZGO/Cr–SiC than that in ZGO/Cr (see the Supporting Information). Due to the fact that the inverse
spinel crystal structure is related to the N2 line, the results of
Raman and FTIR confirm the increased number of N2-type defects in
ZGO/Cr–SiC.</p><!><p>The reaction parameters of the hydrothermal synthesis were evaluated via the near-infrared emission originating from Cr3+ ions, as seen in the PL spectra (integrated between 680–720
nm) in Figure 5a–c.
It is noteworthy that a reaction temperature of at least 200 °C
was needed for detectable luminescence at 10 h of reaction, whereas
the pH had no significant effect on the product in the pH range of
6–10. More specifically, the same amount of precipitate was
formed after 10 h for each pH value, even for acidic pH, without hydroxide
precipitation.</p><!><p>(a) RT-PL intensity vs nominal SiC concentration
for ZGO/Cr–SiC NPs and (b) RT-PL vs nominal
Cr3+ concentration for ZGO/Cr (blue) and ZGO/Cr–SiC
NPs (red). The growth kinetics were studied as a function of (c) emission
intensity, (d) mass concentration, and (e) hydrodynamic size. (f)
Raman spectra of ZGO/Cr–SiC NPs after different reaction times.
The ZGO grown on SiC crystallizes only after 10 h similar to the unseeded
sample.</p><!><p>The emission intensity of Cr3+ PL versus nominal SiC and Cr3+ concentration can be seen in Figure 5a,b, respectively.
The PL intensity gradually increases with increasing SiC concentrations
(Figure 5a) in the
reaction mixture and reaches a plateau at around 1 × 10–5 mol/L [SiC]. The effect of [Cr3+] displayed the same
trend for ZGO/Cr and ZGO/Cr–SiC around the optimal Cr3+ concentration (blue and red curves, as shown in Figure 5b, respectively). It should
be noted that ZGO/Cr–SiC NPs show luminescence even when the
Cr3+ concentration decreased by an order of magnitude,
whereas luminescence of ZGO/Cr NPs was only detected above a nominal
Cr3+ concentration of 1 × 10–5 mol/L.
The Cr3+ content in the NPs was determined by means of
high-resolution AAS for the two nominal concentrations 1 × 10–6 and 1 × 10–7 mol/L. For 1
× 10–6 mol/L [Cr3+], AAS indicated
marginal differences between the samples with and without SiC (see Table 1). However, when the
nominal Cr3+ concentration was reduced to 1 × 10–7 mol/L, the ZGO/Cr–SiC NPs displayed a much
higher Cr3+ concentration than that for the ZGO/Cr NPs.
The very low Cr concentration (as measured by AAS) for ZGO/Cr agrees
well with the undetectable PLfor low (nominal) Cr concentrations (blue
curve, as shown in Figure 3b), while AAS and PL analyses indicate that SiC promotes Cr3+ ions to be built into the ZGO crystal.</p><!><p>Following
PL intensity evolution through the reaction time (Figure 5c), the plot shows a sigmoidal
colloid formation kinetics for ZGO/Cr NPs (blue curve, as shown in Figure 5c). For ZGO/Cr–SiC
NPs (red curve, as shown in Figure 5c), the exponential increase begins after a much shorter
reaction time (6 h), and the overall graph shows a more complex shape
with a double sigmoidal characteristic. We found a very similar trend
for mass concentration (Figure 5d) and hydrodynamic size (Figure 5e). Indeed, the Lifshitz–Slyozov–Wagner
(LSW) model31,32 describes the growth of individual
colloidal particles, assuming that the enlargement rate (i.e., growth kinetics) of colloidal NPs is determined by two mechanisms
including the initial diffusion of precursor species from the bulk
solution to the surface of the growing NP and the following reaction
of the precursor species on the surface of the NP. The colloid particle
formation and growth kinetics are described by a reaction-limited
particle formation at the early stage, followed by fast particle growth
due to the autocatalytic surface reaction and a diffusion-limited
particle growth33 later on. According to
Wu et al.,34 the diffusion
constant can be extracted from the end of the reaction. We obtain
the same slope for the diffusion coefficient determination showing
no differences at the very late (diffusion-limited) stage between
ZGO/Cr and ZGO/Cr–SiC NPs (see the Supporting Information), which is expected when SiC NPs are already covered,
and the oxide grows on the same oxide surface.</p><p>Even though SiC
NPs can catalyze the oxide formation, ZGO/Cr and ZGO/Cr–SiC
have the same particle size after 10 h of reaction with similar crystallinity.
Following the reaction with Raman spectroscopy (Figure 5f), it can be seen that the oxide in ZGO/Cr–SiC
NPs is mainly amorphous at the early stages. Furthermore, the variation
of particle size between 7 and 10 h of reaction is only 18%, and the
sample crystallinity becomes much more pronounced after 10 h of reaction.</p><p>Larger SiC NPs (i.e., ø = 4–6 nm)
do not participate in core–shell structure formation and do
not enhance the luminescence, and the particle growth follows the
same single sigmoidal kinetics as that of the SiC-free sample (see
the Supporting Information), indicating
that the size, or the size-selective properties23,35 of the seeds, has considerable impact on the reaction.</p><!><p>The local environment
of Cr3+ ions shows small differences
when SiC is present in the host NPs (see Figure 4). Indeed, a different local environment
of Cr3+ ions in ZGO/Cr compared to ZGO/Cr–SiC is
a possible explanation for the different PL intensities and the resulting
luminescence enhancement in ZGO/Cr–SiC NPs. However, the n7
defect (whose concentration was found to be higher in the presence
of SiC NPs) usually decreases the emission intensity36 and such a difference cannot explain the excitation wavelength-dependent
enhancement.</p><p>The relative energy positions of the ground and
excited states of the different constituents (i.e., ZGO, Cr3+, and SiC) can reveal the possibility of electron
transfer in the system. To visualize the relative positions of the
ground and excited states, we calculated the energy levels of ZGO/Cr
and ZGO/Cr–SiC from PLE and UPS measurements (see the Supporting Information for more details), as
shown in Figure 6.
The energy positions of the Cr3+ states are also displayed
in Figure 6, and possible
excitation and relaxation pathways between the ZGO/ZGO–SiC
host and Cr3+ ions are indicated by arrows.</p><!><p>Energy levels of ZGO/Cr
and ZGO/Cr–SiC NPs. The energetic
positions of the valence band (VB) onset of ZGO/Cr and SiC were determined
by UPS measurements, and the onset of the conduction band (CB) for
ZGO/Cr was determined by its band gap value (5.2 eV, see the Supporting Information), whereas the CB onset
of SiC was estimated considering its optical band gap, as measured
by PLE. The energetic positions of the Cr3+ states were
determined by PLE. Blue and green arrows indicate different excitation
pathways I–III, whereas non-radiative relaxations and electron/hole
transfer are marked by dashed lines.</p><!><p>In the energy level diagram, it can be seen that the Fermi level
lies just below the 2E excited state of Cr. The energy
difference between the 4A2 ground state of Cr
and the CBMSiC is about 2.4 eV, whereas the energy difference
between VBMZGO/Cr and CBMSiC is about 4.2 eV.
The energy difference of 4.2 eV is consistent with the extra peak
appearing at around 320 nm in the PLE of the ZGO–SiC system
and its disappearance when Cr is present (Figure 3b), suggesting electron transfer from SiC
to Cr3+. The constructed energy diagram does not consider
any distortion caused by the local environment; however, it can be
seen that, upon excitation, the positions of the excited states enable
electron transfer from SiC to Cr3+ since the difference
between CBMSiC and the 4T2 state
of the Cr3+ ion is only 0.2 eV. For excitation energies
larger than the band gap of ZGO (e.g., X-ray and
250 nm UV illumination), the photons can also excite the ZGO host,
enabling new types of relaxation pathways (i.e.,
excitation of VBand core electrons by X-rays), while 290 nm UV illumination
excites only the SiC VBand Cr3+ ions/defects. On the other
hand, SiC and ZGO/Cr create a type-I heterojunction enabling electron
and hole transfer between ZGO/Cr and SiC upon excitation with wavelengths
above the band gap of ZGO. The excitation wavelength-dependent emission
enhancement is supported by quantum yield (QY) measurements of ZGO/Cr–SiC
NPs as a function of the excitation wavelength (see the Supporting Information), where an enhanced QY
was found for wavelengths longer than 260 nm (4.8 eV), corresponding
to the band gap of ZGO.1−3</p><p>It should be noted that the energy-level diagram
can also explain
the weak, broad infrared peaks found in the LT-PL spectrum of ZGO/Cr–SiC
(Figure 4a) because
the energy difference between VBMSiC and the 4A2 ground state (about 1.4 eV) matches these infrared
PL peaks. The reason for the observed doublet feature, as shown in Figure 4a, might be due to
ground-state splitting.</p><!><p>The reaction
mechanisms of the hydrothermal synthesis of ZGO/ZGO/Cr
can be better understood with the results of our detailed reaction
kinetics studies. Even though ZGO synthesis is described as the hydroxide
phase transforming into the corresponding oxide phase via an endothermic dehydration reaction,37 our findings of particle formation without hydroxide precipitation
and correlation with the LSW model, suggest a solution phase reaction
regardless of the presence of the hydroxide. Reaction-limited nucleation
creates an amorphous oxide that crystallizes during the second synthesis
step. The difference in growth kinetics in the presence of SiC NPs
suggests seeded nucleation and growth that does not affect the overall
crystallization step. However, ZGO possesses increased stability on
the surface of the SiC NPs, which results in a more uniform local
environment around the Cr3+ ions in the product.</p><p>Interestingly, seeding via SiC NPs had a minor
effect on the final particle size and yield, which highlights the
importance of the crystallization step in particle formation. The
exothermic reaction probably provides the necessary energy for stable
and rapid NP formation. We rule out the possibility of secondary nucleation
as an explanation of the double sigmoidal kinetics of ZGO/Cr–SiC
NPs because the same mean particle size and dispersity were observed
by DLS and TEM.</p><p>LT-PL and ESR analyses confirmed that SiC NPs
coordinated with
Cr3+ ions and increased the concentration of Cr-cluster-type
defects. On the one hand, this had a minor effect on the optical properties
in the optimal Cr3+ concentration range as the majority
of the Cr3+ ions are in the N2-type local distortion in
either sample. On the other hand, the coordinating effect of SiC NPs
resulted in PL even at low Cr3+ concentration. This indicates
that the Cr-cluster-type defect formation by SiC NPs becomes important
in PL processes for low Cr3+ concentration.</p><p>The core–shell
structure of ZGO/Cr–SiC NPs creates
a type-I heterojunction that, together with the band alignment of
the Cr3+ excited states and the SiC CB, opens up new excitation
pathways (marked by II and III in Figure 6) and channels excitons more efficiently
to the Cr3+ excited states, with significantly increasing
emission efficiency of the system. The ZGO sub-band gap excitation
(e.g., 290 nm UV illumination) only enables SiC–Cr
interactions, which doubled the emission intensity with respect to
ZGO/Cr. Excitation above the ZGO band gap (e.g.,
250 nm UV and hard X-ray illumination), however, enabled ZGO/Cr–SiC
interactions and, due to the heterojunction structure, the emission
intensity was an order of magnitude higher in the presence of SiC.</p><!><p>We found that seeded nucleation using ultrasmall
(<4 nm) SiC
NPs can be used for undoped (ZGO) and Cr-doped ZnGa2O4 (ZGO/Cr) NP hydrothermal growth to improve the structural
and luminescence properties of the particles. The particle size of
the SiC NPs has considerable influence on seeding, that is, only SiC
NPs with diameters below 3 nm act as seeded nucleation sites. The
present growth kinetics study shows that nucleation and crystallization
are different processes, and seeding via ultrasmall
SiC NPs propagates only the nucleation. Furthermore, SiC NPs interact
with Cr3+ ions, which causes an increased concentration
of Cr cluster-type defects in ZGO/Cr–SiC NPs.</p><p>When illuminated
by X-ray light, the ZGO/Cr–SiC NPs showed
remarkable enhancement of Cr3+ ion red luminescence (≈700
nm), namely, by an order of magnitude with respect to ZGO/Cr. A similar
enhancement of red luminescence was found, when 250 nm UV illumination
was used, whereas 290 nm UV illumination enhanced it only by a factor
of 2. A detailed analysis of the electronic properties provides evidence
that the ultrasmall SiC NPs form a type-I heterojunction ZGO/Cr–SiC
nanostructure, promoting the channeling of excitons to the sensitizer
(Cr3+). Such a sensitizing effect by ultrasmall SiC NPs
explains the increased luminescence intensity. We envisage that the
strong response to X-ray light makes the luminescent ZGO/Cr–SiC
NPs potentially promising for in vivo imaging and
X-ray-excited anti-cancer treatments, where high brightness is a fundamental
prerequisite.</p><!><p>SiC NPs with different
diameters, namely, ultrasmall (ø = 1–3 nm) and larger
SiC NPs (ø = 4–6 nm), were synthesized in our laboratory,
and the synthesis and properties, as a function of size, can be found
in our previous reports.23,38,39</p><p>We used nitrates, namely, Zn(NO3)2·6H2O (Sigma, reagent grade, 98%), Ga(NO3)3·xH2O (Sigma, trace metals basis,
99.9%), and Cr(NO3)3·9H2O (Sigma,
trace metals basis, 99%) as cation sources for ZGO synthesis. Aqueous
ammonia solution (32%, VWR, HiPerSolv, CHROMANORM) and HCl (37%, VWR,
Anal-R Normapur) were used for pH adjustment and cleaning. High-purity
18 MÎ© cm Millipore type 1 water (hereafter, DI water) was used
for solvent preparation, dilution, and cleaning. 2-Propanol (IPA)
(VWR, 99.8% HiPerSolv, CHROMANORM) was used for particle precipitation.</p><!><p>Undoped (ZGO)
and chromium-doped ZnGa2O4 (ZGO/Cr), without
and with SiC NPs (ZGO/Cr–SiC), were prepared by a hydrothermal
method based on the report by Li et al.(19) Briefly, 1 mL of Zn(NO3)2, 1 mL of Ga(NO3)3, and 1 mL of Cr(NO3)3 solutions from 2 mol/L Zn(NO3)2, 2 mol/L Ga(NO3)3, and 4 mmol/L Cr(NO3)3 aqueous solutions, respectively, were mixed,
and the total volume was adjusted to 15 mLwith DI water for ZGO/Cr
or an aqueous SiC NP solution (1.5 × 10–5 mol/L) for ZGO/Cr–SiC.
A 2 mL aliquot of ammonium hydroxide (32%) was added to the mixtures
to achieve a pH of 9. After 30 min of stirring, the precursors were
sealed into a PTFE-lined autoclave and annealed at 220 °C for
10 h. The white precipitate obtained after the reaction was centrifuged
out, washed with ammonia solution, DI water, HCl–IPA (0.1 mol/L
HCl and isopropyl alcohol in a 1:10 ratio), and IPA after which it
was dried at 60 °C. Samples were then redispersed in DI water
or pressed into a pellet for characterization. The SiC and Cr3+ concentrations, reaction temperature and time, and pH were
varied in order to find the best synthesis conditions and to study
the reaction kinetics.</p><!><p>We strove for identical sampling
concentrations to accurately compare
the colloid solutions of the different samples via PL, UV–vis,
and XEOL measurements. To achieve that a 5 mg/mL aqueous solution
was prepared by diluting the product of ca. 10 mg/mL.
The weight concentration was measured with a Kern model 770–15
analytical balance from a 1.00 mL sample volume and an MYA 2.4Y microbalance
from a 0.10 mL sample volume. Each measurement was repeated five times.
Due to the smaller density of SiC compared to ZGO, the same particle
size causes some increase in the concentration for ZGO/Cr–SiC.
Considering spherical particles of 9.5 nm diameter with a 2 nm core
diameter, this increase was 11%, which was taken into account.</p><!><p>Author contribution
details; PL spectra of the supernatants
before and after the hydrothermal reaction; XEOL, RT-PL, FTIR, Raman,
UPS, and fitted PLE spectra of ZGO:Cr and ZGO:Cr-SiC NPs; phonon modes
of zinc gallate; an extreme-condition model for quantifying growth
kinetics; quantum yield of ZGO:Cr-SiC NPs as a function of excitation
wavelength; STEM and HRTEM images of ZGO:Cr-SiC; details of characterization
methods including XRPD, XEOL, XPS, UPS, UV–vis spectroscopy,
fluorescence spectroscopy, QY measurements, X-band ESR measurements,
atomic absorption spectrometry, HRTEM, SEM, DLS, and Raman and low-temperature
PL measurements (PDF)</p><p>cm0c04671_si_001.pdf</p><!><p>The manuscript
was written through contributions of all authors. D.B, M.V.N., and
M.T contributed equally.</p><!><p>The authors declare no
competing financial interest.</p><!><p>Full description
of author contribution can
be found in the Supporting Information.</p>