Revealing Interplay of Defects in SnO₂ Quantum Dots for Blue Luminescence and Selective Trace Ammonia Detection at Room Temperature

Abstract

Defect chemistry in SnO2 is well established for resistive sensors but remains to be elusive for photoluminescence (PL) sensors. It demands a comprehensive understanding of the role of cationic and oxygen defects as well as the creation of abundant such defects to provide a selective PL signal. To accomplish it, SnO2 quantum dots (QDs ∼ 2.4 nm) are prepared without a capping agent along with other dimensions. Then, the relationship of defects with the blue-emission PL is unfolded by electron energy loss spectroscopy, lifetime measurements, X-ray absorption, and Raman spectroscopic measurements. The defects acting as Lewis acid sites are utilized for selective ammonia detection. Huge enhancements of the obscured blue luminescence at 2.77 and 2.96 eV from the SnO2 QDs are observed because of interaction with ammonia. The linear variation of PL intensities with analyte concentrations and the recovery of the sensor are elaborated with detection up to 5 ppm. The interplay of defects in SnO2 is further established theoretically for site-specific interactions with ammonia by density functional theory (DFT) calculations. Thus, the unique mechanism revealed for the superlative performance of the PL sensor with uncapped SnO2 QDs provides a novel platform for defect-engineering-based optoelectronic applications.

A Flexible Self‐Powered UV Photodetector and Optical UV Filter Based on β‐Bi2O3/SnO2 Quantum Dots Schottky Heterojunction

“…The existence of oxygen vacancies is most likely to attribute for this crystalline defect. [ 24 ] The bandgap calculated from the PL data was in agreement with the bandgap obtained from Tauc's plot.…”

Single-Molecule Detection-Enabled Plasmonic Ag Nanogap for Unmasking Vibrational Properties in 0D SnO2

“…Particle sizes of ∼2.4 nm for QDs and ∼25 nm of SnO 2 (800 °C) are deduced from the TEM studies, and detailed results are shown in Figure b,c, respectively. Details regarding the particle size distribution and phase conformation from TEM for both SnO 2 QDs and SnO 2 (800 °C) are provided in our previous report . The calculated band gap of SnO 2 QDs is found to be 4.2 eV against a value of 3.6 eV for SnO 2 (800 °C) from the UV–Vis study, as depicted in the Supporting Information (Figure S1).…”

“…The broad peak found in XRD patterns (Figure 2a) of and SnO 2 (800 °C) are provided in our previous report. 7 The calculated band gap of SnO 2 QDs is found to be 4.2 eV against a value of 3.6 eV for SnO 2 (800 °C) from the UV−Vis study, as depicted in the Supporting Information (Figure S1). 37 The incremental band gap for 2.4 nm particles compared with the bulk SnO 2 is attributed to the quantum confinement size effect.…”

“…Raman spectroscopy is widely used to study the unfathomable structural details of tin oxide (SnO 2 ), a technologically important n-type semiconductor, in a nondestructive manner. − Rutile SnO 2 has 18 vibrational modes in the first Brillouin zone. Out of these modes, A 1g , B 1g , B 2g , and E g modes are Raman-active and A 2u and E u modes are infrared (IR)-active. , However, vibrational properties of SnO 2 nanocrystals of less than 10 nm size are considerably affected by highly nonstoichiometric surfaces, and the related influence of the surface on the vibrational property reaches the highest for a particle size equivalent to the Bohr radius (2.7 nm). ,, Because of quantum confinement and a very high surface area, zero-dimensional (0D) SnO 2 with such a dimension is excellent for applications in energy storage, photocatalytic ability, room-temperature optical sensing, and resistive gas sensing. ,− In this direction, Raman spectroscopy is a vastly utilized tool to reveal crucial structural information of SnO 2 nanostructures (NSs), including the disorder and surface defect-related modes. − Interestingly, for SnO 2 quantum dots (QDs), in contrast to theoretically allowed modes, only a broad feature centered at 573 cm –1 (S) is generally reported. The S peak is correlated with the presence of oxygen vacancies ( O V ), which controls various optical and electrical properties. ,,, The origin of the S peak was reported to be related to an amorphous shell that covers a tiny crystalline core, which was intuitively assumed from certain evidence from high-resolution transmission electron microscopy (HRTEM), with no clear proof by the Raman spectroscopy under normal conditions. , Apparently, the Raman signal from the crystalline core is highly subdued under the defect-induced broad peak at 573 cm –1 .…”

“…QDs with an average size of 2.4 nm were prepared by a soft-chemical method, and the detailed process is described in an earlier report. 7 In brief, NH 4 OH was added to an aqueous solution of SnCl 4 , and the resultant white precipitate was washed several times with Millipore water and then dried at 100 °C to obtain QDs. Typical crystalline SnO 2 NPs of ∼25 nm were prepared by annealing QDs at a temperature of 800 °C in an air atmosphere.…”