Red-shifted Photoluminescence Research Articles

Highly Efficient Red/Near-InfraredPhosphorescence from Doped Crystals.

Organic red/near-infrared (NIR) room temperature phosphorescence (RTP) materials with low toxicity and facile synthesis are highly sought after, particularly for applications in biotechnology and encryption. However, achieving efficient red/NIR RTP emitters has been challenging due to the weak spin-orbit coupling of organics and the rapid nonradiative decay imposed by the energy gap law. Here we demonstrate highly efficient red/ NIR RTP with boosted quantum yields (Φp) of up to 32.96% through doping the thionated derivatives of phthalimide (PAI) (MTPAI and DTPAI) into PAI crystals. The red-shifted photoluminescence (PL) stems from a combination of the external heavy atom effect and the formation of emissive clusters centered around electron-rich sulfur atoms. Furthermore, the dopants enhance exciton generation efficiency and facilitate energy transfer from smaller PAI units to larger aggregates, leading to dramatically increasedΦp. This strategy proves universal, opening possibilities for acquiring long-wavelength RTP with tunable photophysical properties. The doped crystals exhibit promising applications in optical waveguides and encryption paper/ink. This research provides a practical approach to obtaining long-wavelength RTP materials and offers valuable insights into the mechanisms governing host-guest systems.

(Invited) Defect Photoluminescence Generation in Boron Nitride Nanotubes by Chemical Functionalization Approaches

Boron nitride nanotubes (BNNTs) are structural analogues of carbon nanotubes (CNTs). They have nanocylindrical structures composed of rolled-up hexagonal boron nitride (h-BN) sheets. The BN framework consists of alternately connecting boron and nitrogen atoms with sp2 hybridization. Therein, a strong covalent bond with an ionic character is formed due to their electronegativity difference. Accordingly, BNNTs exhibit unique properties such as high mechanical strength, thermal stability, and a wide bandgap semiconducting feature [1]. The large bandgap (ca. 6 eV) is less dependent on tube diameters and chiralities, and optical absorption and emission for pristine tubes occur in deep UV regions (around 200 nm) based on the band edge transition. In typical BNNT samples, however, atomic vacancies in the BN framework are reported to provide longer wavelength photoluminescence (PL) in UV–vis regions [2].To date, chemical functionalization of single-walled CNTs (SWCNTs) have been developed to create luminescent defects emitting red-shifted and bright near-infrared PL [3-6]. In this study, we use the chemical functionalization method to produce luminescent defects in BNNTs [7].A reductive alkylation reaction for BNNTs was conducted using sodium naphthalenide and 1-bromohexane to synthesize hexylated BNNTs (h-BNNTs). In a PL spectrum of h-BNNTs, PL peaks appeared at 334 and 413 nm, which were not observed for unmodified BNNTs. X-ray photoelectron spectroscopy revealed that the hexyl group was covalently attached on the boron site of BNNTs, by which sp3 hybridized boron atoms were partly formed in the BN network as defects. The covalent attachment of the hexyl group was also confirmed by atomic-resolution transmission electron microscope observations. Chemical reaction conditions were found to be a key factor for the emission wavelength variations of the resultant defect PL. Therefore, it is revealed that the luminescent defect formation in BNNTs is achieved based on the chemical functionalization approach. The functionalized BNNTs would be applicable to advanced optical applications such as quantum light sources.[1] D. Golberg et al. ACS Nano,4, 2979 (2010). [2] L. Museur et al. J. Appl. Phys., 118, 084305 (2015). [3] T. Shiraki et al. Acc. Chem. Res., 53, 1846 (2020). [4] B. Gifford et al. Acc. Chem. Res., 53, 1791 (2020). [5] A. H. Brozena et al. Nat. Rev. Chem. 3, 375 (2019). [6] J. Zaumseil. Adv. Opt. Mater. 10, 2101576 (2022). [7] T. Shiraki et al. Chem. Lett. 52, 44 (2023). Figure 1

Enhancing photoluminescence of monolayer MoS2 on h-BN substrate through plasmonic resonance for high-power optoelectronic devices

Abstract Hexagonal boron nitride (h-BN) is an excellent substrate material due to its superb thermal conductivity, excellent thermal stability and smooth surface qualities. In this study, a silicon-based h-BN substrate was prepared using the magnetron sputtering technique. Subsequently, monolayer MoS2 obtained by the CVD method was transferred onto the h-BN substrate. Then, Au-core-Ag-shell nanoparticles with a diameter of 80 nm were prepared by a self-assembly method and uniformly dispersed and spin-coated on the MoS2 surface. Applying a laser with a wavelength of 532 nm can effectively excite the plasma resonance effect of the system. Under the intense thermal effect induced by the high laser power and plasma effect, the h-BN substrate ensures that MoS2 exhibits less red-shift and higher photoluminescence intensity compared with SiO2 substrate due to its excellent thermal conductivity. This means that h-BN substrates are particularly suitable for optoelectronic applications under high laser power environments, and can maintain excellent optoelectronic device performance while effectively curbing overheating to ensure long-term stable device operation.

Near-Full-Spectrum Emission Realized in a Single Lead Halide Perovskite across the Visible-Light Region.

The engineering of tunable photoluminescence (PL) in single materials with a full-spectrum emission represents a highly coveted objective but poses a formidable challenge. In this context, the realization of near-full-spectrum PL emission, spanning the visible light range from 424 to 620 nm, in a single-component two-dimensional (2D) hybrid lead halide perovskite, (ETA)2PbBr4 (ETA+ = (HO)(CH2)2NH3+), is reported, achieved through high-pressure treatment. A pressure-induced phase transition occurs upon compression, transforming the crystal structure from an orthorhombic phase under ambient conditions to a monoclinic structure at high pressure. This phase transition driven by the adaptive and dynamic configuration changes of organic amine cations enables an effective and continuous narrowing of the bandgap in this halide crystal. The hydrogen bonding interactions between inorganic layers and organic amine cations (N-H…Br and O-H…Br hydrogen bonds) efficiently modulate the organic amine cations penetration and the octahedral distortion. Consequently, this phenomenon induces a phase transition and results in red-shifted PL emissions, leading to the near-full-spectrum emission. This work opens a possibility for achieving wide PL emissions with coverage across the visible light spectrum by employing high pressure in single halide perovskites.

Lattice-guided assembly of optoelectronically active π-conjugated peptides on 1D van der Waals single crystals.

The deployment of organic molecules in high-performance devices strongly relies on the formation of well-ordered domains, which is often complicated by the dynamic and sensitive nature of supramolecular interactions. Here, we engineered the assembly of water-processable, optoelectronic π-conjugated peptides into well-defined organic-inorganic heterointerfaced assemblies by leveraging the long-range anisotropic ordering of 1D van der Waals (vdW) crystals composed of subnanometer-thick transition metal sulfide chains (MS3; M = Nb, Ta) as assembly templates. We found that the monomers can readily form 1D supramolecular assemblies onto the underlying crystal surface, owing to the structural correspondence between the π-π interactions of the quaterthiophene (4T)-based peptide units (DDD-4T) and sulfur atom ordering along the NbS3 (100) surface. The heterointerfaced assemblies exhibited substantially red-shifted photoluminescence and enhanced visible-range photocurrent generation compared to solution-assembled films. Our results underscore the role of lattice matching in forming ordered supramolecular assemblies, offering an emergent approach to assembling organic building blocks endowed with improved physical properties.

Mechanochemical synthesis of a red luminescent coordination polymer from a polydentate quinoline ligand with large conjugation

Abstract A red luminescent coordination polymer has been prepared from a polydentate 8-hydroxyquinoline ligand via mechanochemical synthesis. The obtained coordination polymer is micro/nano spherical and can suspend stably in organic solvents after ultrasonic treatment. Due to the large conjugation of the ligand with a strong polar s-triazine core, the coordination polymer shows a largely red-shifted emission relative to the ligand. The photoluminescence peak of the coordination polymer in organic solvent is at 613 nm, while a largely red-shifted photoluminescence peak at 675 nm and a shoulder peak at 770 nm are found in the photoluminescence spectrum of the coordination polymer in solid due to the strong interchain interaction.

Ligand‐Induced Digital Programmable Photochromic CdS Materials Toward Dual‐Mode Light‐Printing and Information Encryption

AbstractThe explosive growth of information and its widespread availability underscores the need for robust encryption and anticounterfeiting measures. In this study, CdS quantum dots are engineered (QDs) to manifest multiple visual responses to a single trigger through strategic ligand design. The surface engineering method allows QDs to transition from yellow to black upon photoexcitation‐induced electron transfer from Cd(II) to Cd(0). Surface ligands desorption under hole injection, leading to an increase in QDs size and resulting in a redshift in photoluminescence. This photoexcitation‐induced redox reaction reveals unprecedented photochromism and photoluminescence phenomena, establishing a foundation for advanced information protection measures. Utilizing these QDs, excellent writing performance under UV irradiation is achieved in solid‐state substrates, while a dual‐mode encryption system is realized in gel matrices, opening up new avenues for information encryption as well as cumulative and interactive information protection. Furthermore, the redox reaction of CdS QDs is employed as ink for 3D printing, enabling for the creation of digitally programmable materials with distinct temporally evolving appearances by controlling the oxygen content in the ink to regulate the rate of photochromism. This advancement also sheds light on the progress in 3D printing technology.

Anion-Responsive Colorimetric and Fluorometric Red-Shift in Triarylborane Derivatives: Dual Role of Phenazaborine as Lewis Acid and Electron Donor.

Photophysical modulation of triarylboranes (TABs) through Lewis acid-base interactions is a fundamental approach for sensing anions. Yet, design principles for anion-responsive TABs displaying significant red-shift in absorption and photoluminescence (PL) have remained elusive. Herein, a new strategy for modulating the photophysical properties of TABs in a red-shift mode has been presented, by using a nitrogen-bridged triarylborane (1,4-phenazaborine: PAzB) with a contradictory dual role as a Lewis acid and an electron donor. Following the strategy, PAzB derivatives connected with an electron-deficient azaaromatic have been developed, and these compounds display a distinct red-shift in their absorption and PL in response to an anion. Spectroscopic analyses and quantum chemical calculations have revealed the formation of a tetracoordinate borate upon the addition of fluoride, narrowing the HOMO-LUMO gap and enhancing the charge-transfer character in the excited state. This approach has also been demonstrated in modulating the photophysical properties of solid-state films.

Impact of carrier extraction on photoinduced phase segregation of mixed-halide perovskites

Photoinduced phase segregation (PHS) poses a significant challenge in the practical utilization of mixed-halide perovskites (MHPs) for photovoltaic applications. In this Letter, we investigate the behavior of PHS within operational photovoltaic devices, extending beyond the conventional focus on single-layer MHPs. Our research explores the influence of carrier extraction on PHS by employing various charge transport layers (CTLs) in combination with perovskite films, utilizing photoluminescence (PL) characterization, mainly reflected through the time-dependent PL redshift. Furthermore, we observe that photovoltaic devices capable of efficient carrier extraction exhibit enhanced photoinduced stability during regular operation conditions such as short-circuit operation, as compared to open-circuit conditions. Our findings reveal that efficient photogenerated carrier extraction from the perovskite film mitigates the occurrence of PHS. This work underscores the significance of carrier extraction by CTLs in achieving the efficiency and stability of MHP-based photovoltaic devices.

Ion-pairing assemblies of π-extended anion-responsive organoplatinum complexes

ABSTRACT PtII complexes of π-extended dipyrrolyldiketones were synthesized as anion-responsive π-electronic molecules. The dipyrrolyldiketone PtII complexes exhibited red-shifted absorption and photoluminescence properties. In the solid state, [1 + 1]-type anion complexes formed charge-by-charge ion-pairing assemblies when combined with countercations. Detailed theoretical studies of the packing structures revealed favorable interactions between the planar anion complexes and π-electronic cations. Statement of Novelty PtII complexes of π-extended dipyrrolyldiketones, introducing arylethynyl substituents, in the form of anion complexes exhibited the formation of charge-by-charge assemblies with π-electronic cations via i π– i π interactions.

Direct synthesis of CsPbX3 perovskite nanocrystal assemblies.

Inorganic CsPbX3 (X = Cl, Br, I) perovskite nanocrystals (NCs) possess many advantageous optoelectronic properties, making them an attractive candidate for light emitting diodes, lasers, or photodetector applications. Such perovskite NCs can form extended assemblies that further modify their bandgap and emission wavelength. In this article, a facile direct synthesis of CsPbX3 NC assemblies that are 1 μm in size and are composed of 10 nm-sized NC building blocks is reported. The direct synthesis of these assemblies with a conventional hot-injection method of the NCs is achieved through the judicious selection of the solvent, ligands, and reaction stoichiometry. Only under selective reaction conditions where the surface ligand environment is tuned to enhance the hydrophobic interactions between ligand chains of neighbouring NCs is self-assembly achieved. These assemblies possess narrow and red-shifted photoluminescence compared to their isolated NC counterparts, which further expands the colour gamut that can be rendered from inorganic perovskites. This is demonstrated through simple down-converting light emitters.

(Invited) Formation and Characterization of Fe-Silicide Nanodots for Optoelectronic Application

Light emission from group-IV based nanostructures has been attracting much attention in the field of Si-based photonics because of its potential to combine photonic processing with electronic processing on a single chip [1]. Among various Fe-silicides, semiconducting β-FeSi2 shows light emission in the near-infrared region [2, 3] and is one of promising materials in optoelectronic application from viewpoints of tuning emission energy and improving luminous efficiency by means of quantized structures. In this work, we focus on low temperature formation of β-FeSi2 nanodots (NDs) over ~1011 cm-2 in areal density on SiO2 and the characterization of light emission properties of β–FeSi2 NDs.After conventional wet-chemical cleaning steps of p-type Si(100) substrates, a ~300-nm-thick SiO2 layer was grown at 1000°C in dry O2 and followed by dipping shortly in a dilute HF solution to make uniform surface termination with Si-OH bonds. After that, Fe films in the thickness range from 1.0 to 3.0 nm were formed on the OH-terminate SiO2 layer by electron beam evaporation without any extra heating. And then, the Fe–films were exposed to remote plasma of pure H2 (H2-RP) at 400°C to form highly-dense Fe-NDs [4]. Subsequently, so-prepared Fe-NDs were exposed to pure SiH4 at 400°C while controlling the amount of SiH4 exposure by duration and pressure.AFM images taken after exposure of Fe-films to H2-RP confirm the high-density formation of well-defined NDs. In the cases of H2-RP exposure of 1.3-nm-thick and 3.0-nm-thick Fe-films, the areal dot densities as high as ~1.1 × 1011 and ~1.3 × 1011 cm−2 were obtained, respectively, in which the average dot-heights of self-assembled Fe-NDs were determined to be ~5.3 and ~10.9 nm, respectively, by fitting a log-normal function to each measured size distribution. Notice that AFM images taken after SiH4 exposure show almost no change in the areal dot density while a slight increase in the average dot height. From cross-sectional transmission electron microscope images, we have also verified the formation of distorted-spherical shape NDs with a lattice spacing of ~0.18 nm associated with β-FeSi2. And the result is well consistent with the chemical composition of NDs after SiH4 exposure as evaluated by x-ray photoelectron spectroscopy. In addition, with 976 nm light excitation using a semiconductor laser, the NDs after the SiH4 exposure show stable photoluminescence (PL) signals in the energy region from ~0.7 to ~0.9 eV even at room temperature. On the other hand, no PL signals were detected from the NDs before the SiH4 exposure and after Ar anneal at 400°C. Also, when the size of the Fe-silicide NDs was increased from ~5.3 to ~10.9 nm in average dot height by increasing the initial Fe-film thickness, a distinct red-shift in PL was clearly observed. Thus, the observed PL can be interpreted in terms of radiative recombination of photogenerated carriers through quantized states of β-FeSi2 NDs. These results indicate that SiH4 exposure to Fe-NDs formed on SiO2 is a very promising technique for the low-temperature fabrication of luminescent FeSi2 NDs.AcknowledgementThis work was supported in part by Grant-in-Aid for Scientific Research (A) 21H04559 of MEXT Japan, and the Cooperative Research Project Program of the Research Institute of Electrical Communication, Tohoku University.

Regulation of aggregation-induced emission and reversible remarkable mechanofluorochromism by substitution position of the carbazole unit in D-A type carbazole-functionalized dicyanoethylene π-systems

In this study, two twisted D-A type dibenzothiophene- and carbazole-functionalized dicyanoethylene derivatives were formulated and synthesized. They were labeled as 2-substituted carbazole (BT-CE-C2) and 3-substituted carbazole (BT-CE-C3), respectively. Furthermore, an investigation into the correlation between the molecular structure of these derivates and their photophysical properties was conducted. The characteristic D-A structures and the highly distorted spatial conformations of the two fluorophores led to the emergence of a unique intramolecular charge transfer (ICT) effect and remarkable aggregation-induced emission (AIE) behavior. Notably, the position of substitution of the carbazole unit regulated the reversible high-contrast mechanofluorochromic (MFC) properties demonstrated by BT-CE-C2 and BT-CE-C3. The as-prepared solid powders of BT-CE-C2 and BT-CE-C3 produce blue-green and orange-yellow emissions at 508 and 567 nm, respectively. However, their ground powders emitted orange (at 589 nm) and orange-red (at 619 nm) lights. Notably, BT-CE-C2 exhibited enhanced fluorescence upon external stimulation, increasing its solid-state luminescence efficiency from 0.035 to 0.189. In contrast, the BT-CE-C3 exhibited a decrease in fluorescence, causing its solid-state luminescence efficiency to change from 0.426 to 0.348. The fluorescence lifetimes, powder X-ray diffraction (PXRD), and spectral analysis collectively identified the phase transition between the crystalline and amorphous states as the underlying cause of the MFC behavior observed in BT-CE-C2 and BT-CE-C3. Additionally, the red shift in photoluminescence (PL) spectra under external stimulus can be attributed to planarized-induced charge transfer (PICT), exciton coupling, increased π-π interactions, and enhanced orbital overlap between adjacent molecules. This combination of factors led to the observed MFC properties of these compounds.

Distyrylbenzene-based semi-conducting molecular material for OLED applications: Synthesis, characterization and TD-DFT calculations

New semi-conducting molecular materiel (M-DSB) based on the distyrylbenzene π-conjugated system was synthesized via the Wittig reaction. The structural properties of M-DSB, displaying three different stereoisomers E-E, Z-Z and Z-E, were investigated through density functional theory (DFT) optimization. The detailed TD-DFT analysis provided insights into the structure-property relationships and revealed good agreement with experimental data. This photoactive material exhibits nearly the same UV–visible spectra in dilute solution and as solid thin film, with a maximum absorption at 363 nm and an optical gap of 2.99 eV. A broad and red-shifted photoluminescence spectrum was obtained in the solid state with blue emission compared to the solution which shows purplish blue emission with relatively narrow emission. The HOMO/LUMO energy levels were calculated by cyclic voltammetry measurements and indicate a p-type semiconductor material. The temperature and frequency dependence of the conductivity shows an activation energy value of about 0.89 eV for the M-DSB thin layer. Impedance spectroscopy of the ITO/M-DSB/Al device indicates a typical behavior of a single relaxation process and confirms that the electrical response becomes faster and the mobility of the charge carriers is enhanced by increasing the temperature.

Photoluminescence Redshift of AgInS<sub>2</sub> Quantum Dots by Employing Shells with Graded Composition

Photoluminescence Redshift of AgInS<sub>2</sub> Quantum Dots by Employing Shells with Graded Composition

Near-infrared photoluminescent detection of serum albumin using single-walled carbon nanotubes locally functionalized with a long-chain fatty acid

Serum albumin (SA) is a protein found in biofluids that is useful for diagnosing kidney and liver-related diseases. Single-walled carbon nanotubes (SWCNTs) showing photoluminescence (PL) in the near-infrared (NIR) region are prospective candidates for diagnosis NIR probes. Their NIR PL enhancement has been reported by defect introduction techniques based on local chemical functionalization of SWCNTs. The locally functionalized (lf-)SWCNTs exhibit a bright defect PL (>1000 nm) from the defect sites and its sensitive wavelength shifts in response to surrounding dielectric atmospheres have been reported. Here, we used the sensitive defect PL responses of lf-SWCNTs to establish a NIR sensing system for SA detection. A palmitic acid group that binds to SA strongly and selectively, was functionalized at the defect site of lf-SWCNTs (lf-SWCNTs-p). Compared to typical PL of unmodified SWCNTs, selective defect PL red-shifts (of 2.5 nm max.) were clearly observed according to the addition of SA in phosphate-buffered saline. The defect PL responsiveness could detect different SA from human, bovine, and mouse. The red-shifts of the defect PL occurred by formation of a high dielectric environment based on the specific binding between SA and the palmitic acid groups on the defect sites. The lf-SWCNTs-p detected SA-spiked serum and albuminuria of diabetic mouse in body fluids. The findings show that the lf-SWCNTs-p offer an NIR PL analytical tool for SA in body fluids applicable to a disease diagnosis and expand the bioapplications of lf-SWCNTs based on their molecular design-based defect engineering.

Kinetic Suppression of Photoinduced Halide Migration in Wide Bandgap Perovskites via Surface Passivation.

In this work, we study the kinetics of photoinduced halide migration in FA0.8Cs0.2Pb(I0.8Br0.2)3 wide (∼1.69 eV) bandgap perovskites and show that halide migration slows down following surface passivation with (3-aminopropyl) trimethoxysilane (APTMS). We use scanning Kelvin probe microscopy (SKPM) to probe the contact potential difference (CPD) shift under illumination and the kinetics of surface potential relaxation in the dark. Our results show that APTMS-passivated perovskites exhibit a smaller CPD shift under illumination and a slower surface potential relaxation in the dark. We compare the evolution of the photoluminescence spectra of APTMS-passivated and unpassivated perovskites under illumination. We find that APTMS-passivated perovskites exhibit more than 5 times slower photoluminescence red-shift, consistent with the slower surface potential relaxation as observed by SKPM. These observations provide evidence for kinetic suppression of photoinduced halide migration in APTMS-passivated samples, likely due to reduced halide vacancy densities, opening avenues to more efficient and stable devices.

Layer‐By‐Layer Printed Metal Hybrid (Cs:FA)PbI3 Perovskite Nanocrystal Solar Cells

AbstractMixed halide perovskite nanocrystals in the form of cesium/formamidinium lead triiodide ((Cs:FA)PbI3) offer great potential for efficient and stable solar cells. To date, large‐scale production with roll‐to‐roll compatible deposition methods remains difficult and requires detailed research on each involved processing step. Here, a proof‐of‐concept study about slot‐die coating (printing) the active layer of (Cs:FA)PbI3‐based nanocrystal solar cells is presented. Structural and morphological changes during ligand exchange of long‐chain oleic acid and oleylamine by Pb(NO3)2, and top‐layer FAI passivation are investigated. Ligand exchange improves the processability of the nanocrystal layer and enhances charge transport. It also changes texture from face‐on toward edge‐on orientation as grazing‐incidence X‐ray scattering studies indicate. Ligand exchange and FAI passivation redshift photoluminescence and prolong charge carrier lifetime in the printed nanocrystal films. The proof‐of‐concept feasibility of printing metal halide perovskite nanocrystal films for solar cells is shown by building 20 devices with a median power conversion efficiency of 6.39%.

Lattice Vacancy Induced Energy Renormalization of Photonic Quasiparticles in Two-Dimensional Semiconductors.

Coulomb interactions among dense charges and quasiparticle energy renormalization are at the center of quantum science because they significantly reshape the fundamental electronic and photonic properties of materials. While lattice vacancies are ubiquitous in solid materials, their physical effect on the Coulomb interaction among quasiparticles is normally weak and negligible. Here we show that in atomically thin semiconductors the presence of lattice vacancies emerges as an important but unexplored origin for the nontrivial renormalization of quasiparticle binding energies, due to the subtle modification of overall dielectric functions at low dimensionality. Such a renormalization effect leads to unusual reduction in the energy scales of photonic quasiparticles and red shifts of photoluminescence as the density of lattice vacancies increases. With strict configurative form factors derived, a dielectric screening model is also established for the generalized trilayer systems to capture the fine modification in the energy scales of quasiparticles and to elucidate the dielectric functions versus realistic Bohr lengths. This finding highlights the essential but commonly neglected role of lattice vacancies and deciphers the longstanding enigma of unpredictable photoluminescent line shifts in low-dimensional systems.

Fine‐Tuning Blue‐Emitting Halide Perovskite Nanocrystals

AbstractLead halide perovskite nanocrystals (NCs) with narrow, bright emission in the visible range are promising candidates for light‐emitting applications. Near‐unity quantum yields have been realized for green and red‐emitting perovskites, but efficient, stable blue‐emitting perovskite materials are scarce. Current methods to synthesize quantum‐confined CsPbBr3 NCs with blue emission are limited to specific wavelength ranges and still suffer from inhomogeneously broadened emission profiles. Herein, anisotropic blue‐green emitting CsPbBr3 NCs are synthesized in ambient atmosphere using a spontaneous crystallization method. Optical spectroscopy reveals a gradual, asymptotic photoluminescence (PL) redshift of pristine colloidal NCs after synthesis. During this process, the emission quality improves notably as the PL spectra become narrower and more symmetric, accompanied by a PL intensity increase. Electron microscopy indicates that the gradual redshift stems from an isotropic growth of the CsPbBr3 NCs in at least two dimensions, likely due to residual precursor ions in the dispersion. Most importantly, the growth process can be halted at any point by injecting an enhancement solution containing PbBr2 and organic capping ligands. Thus, excellent control over NC size is achieved, allowing for nanometer‐precise tunability of the respective emission wavelength in the range between 475 and 500 nm, enhancing the functionality of these already impressive NCs.