CO2 Pressure‐Induced Self‐Trapped Excitons in SrTiO3

With strong electron–phonon coupling, self‐trapped excitons (STEs) are typically formed in perovskite materials, and radiative recombination of STEs can produce broadband emission with large Stokes shifts. STEs are essential to further improve the optoelectronic properties of materials. Surprisingly, 2D system is the edge case, with low even no self‐trapping barriers, leading to effortless formation of STEs. In this work, 2D strontium titanate (SrTiO3) with defects is prepared using supercritical carbon dioxide (SC CO2) and its carrier transport and transition are studied. The appearance of wide photoinduced positive absorption signals in the femtosecond transient absorption spectra is direct evidence for the formation of STEs. The presence of STEs is further supported by the increased Stokes shift and full width at half maximum in the steady‐state photoluminescence spectra.


Introduction
Nowadays, the research focusing on photoelectric materials with high performance is facing the emergence of some new changes, among which perovskite materials have received extensive attention due to their outstanding photoelectric properties. [1]Halide perovskites have excellent charge-transfer properties and are greatly utilized in photovoltaic devices such as solar cells and light-emitting diodes. [2]Among them, Pb-Br or Pb-Cl perovskites, which integrate high brightness and tunable emission bandwidths, are ideal for next-generation advances in lighting and display technology as well as substantial energy savings. [3]his advantage arises from exciton self-capture associated with strong exciton-phonon coupling.Strong exciton effects in the soft and nontonal lattices of perovskites with significant quantum and dielectric confinement lead to the formation of self-trapped excitons (STEs) and transient lattice deformation. [4]Additionally, radiative recombination of STEs can produce broadband emission with large Stokes shift, which is essential for further improving the optoelectronic properties of perovskite materials and increasing the application potential of optical emitters. [5]n particular, the 2D perovskites have a soft lattice, dynamically disordered framework [6] and strong electron-phonon coupling. [7]Thus, localized lattice distortions were prone to occur, leading to self-trapped states within the bandgap [8] and forming STEs with emission peaks lower than the emission peaks of free excitons.
Recently, the phenomenon of pressureinduced emission in low-dimensional perovskites has been widely investigated, which mainly involves the modulation of exciton recombination.Zou et al. [9] found that the suppression of lattice distortions and the enhancement of the electron dimension in the excited state play an essential role in the formation of stable bright STEs after applying an external pressure at the GPa level on some halide perovskites.It is known that strong electron-phonon coupling and low electron dimension are essential for the formation of STEs, which are positively correlated with low crystal dimension and octahedral distortion.
Strontium titanate (SrTiO 3 ) is a prime example of perovskites with distinguished optoelectronic properties and a dominant material in the new field of composite-oxide-based electronics. [10]t has a simple cubic structure with an indirect bandgap of 3.27 eV at room temperature and exhibits a rich diversity of electrical properties ranging from insulating to metallic, semiconductor, and superconductor behavior. [11]In this work, SrTiO 3 was selected to study its optical properties by supercritical carbon dioxide (SC CO 2 )-induced phase engineering.Optical characterizations using steady-state photoluminescence (PL) spectra, UV-vis absorption spectra, and time-resolved PL (TRPL) spectra reveal that the Stokes shift and full width at half maximum (FWHM) of the samples are increased after SC CO 2 treatment.The broad and bright photoinduced positive absorption (PIA) signals in the transient absorption (TA) spectra are direct evidence for the generation of STEs.Furthermore, the results of transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) indicate that lattice distortion and the presence of Sr/O vacancies are crucial for the formation of STEs.

Morphology and Structure Characterization
Figure 1 shows SrTiO 3 samples with morphological characterizations and lattice structure under different SC-CO 2 -treated conditions.As shown in Figure 1a-c, a low-magnification TEM image with a slightly transparent single after SC CO 2 treatment, indicating that few-layered SrTiO 3 nanosheets were obtained.The inset of Figure 1b,c illustrates a local enlargement at the red box.In nanoscale, the black and white structures at the edges of SrTiO 3 samples are observed in TEM images of multiple samples with SC CO 2 treatment, considered to be the moiré fringes, which means that a local crystal structural distortion might occur. [12]According to the previous theory and research literatures, the phenomenon of Moiré fringes is related to lattice distortion at the corresponding position, but the exact type cannot be determined by the Moiré fringes.The diffraction highlight in the inset of Figure 1d,e is consistent with (110) plane of SrTiO 3 .The d spacing calculated from the high-resolution TEM (HRTEM) image in Figure 1b that was 0.281 and 0.283 nm (Figure 1f,g), which were slightly increased comparing to the standard SrTiO 3 , indicating that SrTiO 3 is stretched over the SC CO 2 treatment.In addition, some low signal strengths (red box) can be observed from the line profile plot measuring the lattice spacing, indicating that there are vacancies on the surface of SrTiO 3 under SC CO 2 treatment. [13]urthermore, a detailed study is carried out to verify the perovskite structural transformation of the SrTiO 3 by XRD, as shown in Figure 1h.The results show that the diffraction peaks are exactly same as the standard card (PDF#84-0444) with Pm-3m space group, [14] which can be clearly derived to the cubic perovskite phase.Notably, partial enlarged detail of the XRD spectrum (Figure 1i,j) displayed that the diffraction peaks of the samples after SC CO 2 treatment were shifted to the lower angle, indicating that the lattice spacings of SrTiO 3 were increased, corresponding to the results measured in the HRTEM image.The appearance of peak crack in SrTiO 3 under SC CO 2 treatment is key evidence for the phase transition originated from stretching over the SC CO 2 treatment. [15]

Lattice Defect Characterization
XPS spectra is the most common and effective method to determine the contents of each element.In this work, XPS spectra are used to measure the content range of Sr, Ti, and O in samples.
First, the investigation of the survey full XPS spectrum shows the presence of distinct Sr 3d, Ti 2p, O 1s, and C 1s peaks.No other impurities other than the small carbon peak were detected, as shown in Figure S1, Supporting Information.The O 1s XPS spectra (Figure 2a) can be fitted to three Gaussian peaks at 531.6, 530.5, and 528.7 eV, corresponding to OH À adsorbed on the SrTiO 3 surface, O vacancies (O V ) in oxygen defects on the SrTiO 3 surface, and O 2À ions in the O-Ti-O lattice in the SrTiO 3 crystal structure, [16] respectively.Among them, the spectral proportion of 530.5 eV in the SrTiO 3 samples under SC CO 2 treatment was about 2.3 and 3.1 times higher than that of the ultrasonic sample, respectively.In addition, the XPS spectra of Ti 2p (Figure 2b) show two strong Ti 2p 3/2 (≈458.4eV) and Ti 2p 1/2 peaks (≈464.2eV), which are typical Ti 2p core-scale XPS spectra of SrTiO 3 with Ti 4þ properties. [17]For SC-CO 2treated samples, there is also a signal attributed to Ti 3þ , which can be detected at weak peaks with binding energies close to 457.2 eV (Ti 2p 3/2 ) and 463.0 eV (Ti 2p 1/2 ).From Figure 2a,b, it can be seen that the concentration of O V and Ti 3þ on the SrTiO 3 surface increased with the increase of CO 2 pressure.According to XPS spectrum of Sr 3d in Figure 2c, the peaks at 132.5 eV (Sr 3d 5/2 ) and 134.2 eV (Sr 3d 3/2 ) are attributed to Sr 2þ ions, [18] while the peaks at 133.3 eV (Sr 3d 5/2 ) and 135.0 eV (Sr 3d 3/2 ) are consistent with the reported values for SrCO 3 . [19]The appearance of SrCO 3 peaks suggests the formation of defective SrTiO 3 over the SC CO 2 treatment.
To supplement the defect information, Figure 2d shows the electron paramagnetic resonance (EPR) spectra of SrTiO 3 samples, which are applied to further analyze the content of O V and Ti 3þ .The results showed that the samples treated with SC CO 2 showed a stronger O V signal [20] (g = 2.003).In addition, corresponding to g = 1.979, [21] which is assigned to Ti 3þ , it also exhibits a slight enhancement, which is consistent with the analysis of Ti 3þ and O V in XPS peaks.
Raman spectrum is an effective tool for studying structural disorder and local symmetry changes.Inspection of Raman spectroscopy (Figure 2e) shows that a spectral line at 137 cm À1 corresponds to the vibration mode of SrCO 3 , [22] which fully explains that SrCO 3 is generated during CO 2 treatment.In addition, there is an excellent match between the Raman shift of peaks at ≈175, 265, 474, 547, and 795 cm À1 and the frequencies of TO2, TO3, LO3, TO4, and LO4 phonons, [23] respectively, which indicates the presence of first-order vibrational modes in SC-CO 2 -treated samples.This is due to the presence of defects such as lattice distortion and oxygen vacancies, the central symmetry is broken, and first-order vibration mode occurs. [24]

Optical Properties
Femtosecond TA (fs-TA) spectroscopy can be used as a precise approach to expounding the exciton and recombination process of photoinduced carrier. [25]In TA measurements, the PIA change (ΔA) was recorded by the spectrometer as a function of both wavelength and delay time to obtain detailed information on the photoinduced carrier dynamics.Under a photoexcitation wavelength of 325 nm, the SC-CO 2 -treated SrTiO 3 exhibited wideband PIA signals (Figure S2, Supporting Information).The PIA signal may be generated by a combination of multiple mechanisms such as PIA of excitons and exciton-exciton interaction, etc. Self-trapping excitons (STEs) have been reported in previous studies to produce wider PIA signals. [9,26]Therefore, our TA data provide the most direct evidence for exciton self-trapping.
In addition, there are the same identical raise process in PIA band at different wavelengths with a ultrafast time of about 450 fs, consistent with other reported results, [25] indicating the rapid formation process of STEs.One of the most significant features is that obvious positive and negative TA signals are detected in the range of 0-450 fs (insets of Figure 3a-c), and only in the wavelength of ≈775 nm of probe photons, as shown in Figure 3a-c.In this work, the slow decay time at ≈775 nm can be assigned to the relaxation of STEs to study the formation of STEs in SrTiO 3 .In addition, as shown in Figure S3, Supporting Information, when the delay time is 262 fs, two absorption TA singles appear on the spectrum of SrTiO 3 under ultrasonic treatment, which appear simultaneously at ≈760 and ≈795 nm pointed by the arrow, indicating that the exciton carriers with different E b appear in SrTiO 3 . [27]The simultaneous occurrence of absorption peaks at two locations is also one of the characteristics of STEs. [27]Moreover, the formation process of STEs is shown in Figure S4, Supporting Information.The TA data of SrTiO 3 under SC CO 2 treatment display that excited-state absorption occurs at ≈300 fs followed by immediate excited radiation, which is the process from carrier generation to recombination, and finally STEs were produced. [28]With the increase of delay time, the STE-PIA signal of SrTiO 3 under SC CO 2 treatment becomes stronger and finally maintains a positive absorption signal quickly, while the positive signal appears after a negative oscillation of the ultrasonic sample, whose signal strength is lower than SC-CO 2 -treated SrTiO 3 .
We performed multiexponential curve fitting to trace the TA dynamics.The PIA decay signal of ultrasonic sample at ≈578 nm was divided into three distinct processes (Figure 3d): two ultrafast components with lifetime of τ 1 ≈ 5.78 and τ 2 ≈ 5.79 ps and a slow decay process with lifetime of τ 3 ≈ 0.49 ns, while the lifetime of the slow process in SrTiO 3 under SC CO 2 treatment was about 1.73 (τ ≈ 0.86 ns) and 3.12 (τ ≈ 1.56 ns) times higher than that of the ultrasonic SrTiO 3 , respectively.Furthermore, under ultrasonic treatment, the PIA decay signal at ≈722 nm was also divided into three distinct processes (Figure 3e): two ultrafast components with lifetime of τ 1 ≈ 5.20 ps and τ 2 ≈ 6.58 ps and a slow decay process with lifetime of τ 3 ≈ 0.91 ns.While under SC CO 2 treatment with  14 MPa, due to the deepening of defect state and the increase of defect density, the lifetime was composed of two fast processes (τ 1 ≈ 12.38 ps, τ 2 ≈ 12.52 ps) and a slow decay process (τ 3 ≈ 2.29 ns), which are longer than the initial state.The fast process is related to volume defect capture and surface defect capture, corresponding to the non-radiative transitions of trapped carriers, while the slow component is attributed to non-radiative recombination of STEs. [29]In addition, the decay lifetime at ≈775 nm (Figure 3f ) is similar to that at ≈722 nm, and the lifetime values are shown in Table 1.The prolongation effect indicates that the rapid formation of STEs is due to the pressure-driven excited state caused by lattice distortion.
The optical absorption range is investigated by UV-vis absorption spectrum as shown in Figure 4a.We found that all samples possess strong absorption peaks in the ultraviolet region.Among them, SrTiO 3 samples under SC CO 2 treatment have stronger absorption intensity in the ultraviolet region and have weak absorption in the visible region of 400-700 nm.In addition, as shown in Figure 4b, the semiconductor bandgap value is calculated according to the Kubelka-Munk formula [27] : where α, h, v, E g , and A are absorption coefficient, Planck constant, optical frequency, bandgap, and a constant, respectively.Here, n is 2 in this article because of the SrTiO 3 with an indirect bandgap value.The results show that the maximum value (E g ) of the bandgaps is 3.02 eV of SrTiO 3 under SC CO 2 treatment which is 0.05 eV higher than ultrasonic sample, suggesting the bandgap can be considerably enhanced by carrying out pressure engineering.
The PL emission signal is derived from the recombination of photoinduced carriers, which could estimate the charge separation behavior of SrTiO 3 .PL spectra (Figure 4c) of three samples were comparatively investigated at an excitation wavelength of 320 nm.We found that both emission peaks at ≈400 nm of SrTiO 3 under SC CO 2 treatment showed a redshift, indicating that the Stokes shift slightly increased.Furthermore, the CO 2treated SrTiO 3 had another wider emission peak with high intensity at ≈470 nm, and this peak with a large Stokes shift could be assigned to the STE.In addition, the FWHM of the PL peak at ≈400 nm also displays broadening trend significantly.Both of phenomena are key evidence for the existence of STEs. [25]tudying the dynamics of photo-generated charge carriers is crucial to reveal the mechanism, [27] so we performed TRPL measurements, as shown in Figure 4d.The PL-decay lifetime of SrTiO 3 is prolonged under SC CO 2 treatment, which is consistent with the results of TA spectrum.

Conclusion
In summary, we have successfully realized the generation of STEs in SrTiO 3 under photoexcitation using SC CO 2 .The wide PIA signals in the fs-TA spectra and the increased Stokes shifts and FWHM in the steady-state PL spectra are strong evidence for the existence of STEs.The results of experimental studies such as TEM, XRD, XPS, and EPR clearly show that there is a phase transition from cubic phase to tetragonal phase in the lattice structure of SrTiO 3 , and there are even a large number of vacancies on the crystal surface.More importantly, for the first time, it is found out that SC CO 2 and its pressure is an important and effective approach to modulated lattice distortion and defects, which can help produce STEs in SrTiO 3 successfully.Therefore, it can be anticipated that this founding will supply an efficient platform for fabrication of perovskite materials with outstanding photoelectric properties in application of solar cells and light-emitting diodes in the near future.

Experimental Section
Materials: Strontium titanate powder (99%) was purchased from sigma-Aldrich, and directly used without further purification.CO 2 with a purity of 99.99% was purchased from the Zhengzhou Shuangyang Gas Co. Ltd., and deionized water was prepared with double-distilled water.
Preparation of Ultrasonicated SrTiO 3 : The 60 mg bulk SrTiO 3 was dispersed in 30 mL of ethanol/water (V ethanol :V water = 1:1) solution and subjected to ultrasonic treatment for 4 h.The resulted suspension was labeled as ultrasonicated SrTiO 3 .
CO 2 -Induced Phase Engineering: Ultrasonicated SrTiO 3 suspension was directly transferred into the SC CO 2 apparatus composed mainly of a stainless-steel autoclave with a heating jacket and a temperature controller.The autoclave was heated to designated temperature (120 °C), and then CO 2 was charged into reactor to the desired pressures (12/ 14 MPa) and maintained for 4 h under continually stirring.After the CO 2 was slowly released, the supernatant was collected by centrifugation at 3000 rpm for 8 min and the precipitates was collected from the supernatant at 10 000 rpm for 10 min.Finally, the precipitates were dried in the oven at 60 °C.
Characterizations: TEM images were recorded on an FEI Tecnai G2-F20 at an acceleration voltage of 200 kV.The thickness of nanosheets was measured by atomic force microscope (Bruker Dimension Icon).XRD patterns were collected on a Bruker D8 Focus diffractometer (Bruker AXS, Germany) using Cu K radiation.XPS was performed using Thermo Scientific K-αþ system.Raman measurements were performed using LabRAM HR Evolution with laser wavelength of 532 nm.The EPRs were obtained by EPR spectrometer (EMX-9.5/12).The TA measurements were performed with the fs-TA spectrometers described in the previous reports.In combination with an amplified femtosecond laser system (Coherent Inc., the United States) with the pulse width of 25 fs, the fs-TA data were recorded on a Helios pump-probe system (Ultrafast Systems LLC., the United States).In this work, wavenumber of pump pulses was 325 nm (≈40 nJ pulse À1 at the sample), which were delivered by an optical parametric amplifier (TOPAS-800-fs).The stable white-light continuum probe pulses were 400-800 nm for this work generated by focusing the fundamental 800 nm beam into a sapphire plate.The visualized data were finally processed by the Surface Xplorer software.Specific test details can be seen in references.UV-vis absorption spectra spectra were measured on a UV-vis spectrophotometer (UV-3600plus).Steady-state and transient-state PL spectra were recorded on a fluorescence spectrometer (FLS1000/FLS980, Edinburgh Instruments Ltd., the UK).

Figure 3 .
Figure 3. TA measurements on SrTiO 3 with pumping at 325 nm.The 3D plot of the TA spectra of a) ultrasonic, b) 12 MPa, and c) 14 MPa.The insert shows the TA spectra in the range of 0-1 ps near 775 nm.The comparison of TA kinetics of three sample in the delay range of 0-7450 ps: d) ≈578 nm, e) ≈722 nm, and f ) ≈775 nm.

Figure 4 .
Figure 4. Optical characterizations: a) UV-vis absorption spectra of the three samples, b) the (αhv) 1/2 versus hv curve of the samples based on (a), c) PL spectra of the samples, and d) TRPL-decay spectra of the related samples.

Table 1 .
The attenuation life of samples at different absorption centers.