Nanocrystal Films Research Articles

Reversible Ligand Detachment from CdSe Quantum Dots Following Photoexcitation.

The nanocrystal-ligand boundaries of colloidal quantum dots (QDs) mediate charge and energy transfer processes that underpin photochemical and photocatalytic transformations at their surfaces. We used time-resolved infrared spectroscopy combined with transient electronic spectroscopy to probe vibrational modes of the carboxylate anchoring groups of stearate ligands attached to cadmium selenide (CdSe) QDs that were optically excited in solid nanocrystal films. The vibrational frequencies of surface-bonded carboxylate groups revealed their interactions with surface-localized holes in the excited states of the QDs. We also observed transient and reversible photoinduced ligand detachment from CdSe nanocrystals within their excited state lifetime. By probing both surface charge distributions and ligand dynamics on QDs in their excited states, we open a pathway to explore how the nanocrystal-ligand boundary can be understood and controlled for the design of QD architectures that most effectively drive charge transfer processes in solar energy harvesting and photoredox catalysis applications.

Synthesis, Structural Characterization and Temperature Effects on Chloroform Vapor Sensing Properties of ZnO Nanocrystals

In this study, nanocrystalline ZnO were synthesized onto glass substrates by using sol-gel method. Structural characterizations were performed by XRD, SEM and AFM techniques. Potential application of nanocrystal ZnO as chloroform sensor was investigated. Response of the fabricated thin films of the ZnO nanocrystals towards chloroform vapor was investigated depending on gas concentration (750-15000 ppm) between the temperatures of 22‒150 ºC in nitrogen ambient. Gas flow rates were controlled by using flow controller and flow meters. All the measurement system was computerized. XRD results revealed that thin film of the ZnO nanocrystals on the glass substrate was in crystal form and can be characterized by 036-1451 JCPDS number. Crystallite sizes of the ZnO nanocrystals were determined both by SEM images and the Scherrer equation. The crystallite sizes were calculated between 27.9 – 50.4 nm using the Scherrer equation. The sensors showed reversible response towards the chloroform vapor in the measured temperature and gas concentration range. Response time and sensitivity values of the sensors towards the chloroform vapor were also calculated. The increase in temperature caused to increase in sensitivity values. The best sensitivity values were obtained at 150 ºC.

Organosilicon-Based Ligand Design for High-Performance Perovskite Nanocrystal Films for Color Conversion and X-ray Imaging.

Perovskite nanocrystals (PNCs) bear a huge potential for widespread applications, such as color conversion, X-ray scintillators, and active laser media. However, the poor intrinsic stability and high susceptibility to environmental stimuli including moisture and oxygen have become bottlenecks of PNC materials for commercialization. Appropriate barrier material design can efficiently improve the stability of the PNCs. Particularly, the strategy for packaging PNCs in organosilicon matrixes can integrate the advantages of inorganic-oxide-based and polymer-based encapsulation routes. However, the inert long-carbon-chain ligands (e.g., oleic acid, oleylamine) used in the current ligand systems for silicon-based encapsulation are detrimental to the cross-linking of the organosilicon matrix, resulting in performance deficiencies in the nanocrystal films, such as low transparency and large surface roughness. Herein, we propose a dual-organosilicon ligand system consisting of (3-aminopropyl)triethoxysilane (APTES) and (3-aminopropyl)triethoxysilane with pentanedioic anhydride (APTES-PA), to replace the inert long-carbon-chain ligands for improving the performance of organosilicon-coated PNC films. As a result, strongly fluorescent PNC films prepared by a facile solution-casting method demonstrate high transparency and reduced surface roughness while maintaining high stability in various harsh environments. The optimized PNC films were eventually applied in an X-ray imaging system as scintillators, showing a high spatial resolution above 20 lp/mm. By designing this promising dual organosilicon ligand system for PNC films, our work highlights the crucial influence of the molecular structure of the capping ligands on the optical performance of the PNC film.

Comparative study of the near-infrared detection of PbSe fabricated using a sputter deposition method

Near-infrared (NIR) light is used in nondestructive spectroscopies to analyze various compounds in several applications. A PbSe nanocrystal film used in NIR detectors is fabricated using a magnetron sputtering process. The PbSe NIR detectors can be fabricated at a low cost and can operate at ambient temperatures. In this study, the surface morphology and chemical and crystal states of PbSe and its NIR detection performance are investigated and compared with existing reports in the literature. The PbSe film grown by sputtering exhibits nano- and columnar crystal structures, predominantly halite-like (100) face-centered cubic structures. The PbSe film contains PbO and SeO2 and exhibits dual-absorption band-to-band transitions at 0.43 and 0.52 eV. The sputtered PbSe film is used to fabricate an NIR photodetector with a metal-semiconductor-metal structure. The detector shows fast rise and decay times of 0.14 and 0.12 s, respectively, and responsivity of approximately 0.01 A/W under infrared illumination. Based on this study, a low-cost PbSe NIR detector that can operate at room temperature is developed for early fire detection.

Sodium-alginate-based indicator film containing a hydrophobic nanosilica layer for monitoring fish freshness

In this study, hydrophobic sodium alginate/anthocyanin/cellulose nanocrystal indicator films were fabricated by incorporating nanosilica (NS) as a waterproofing layer. The concentrations and formation methods (spraying (S), coating (C), and impregnation (I)) of the NS layer (denoted as NSS, NSC, NSI, respectively) were optimized. The results indicated that the optimum concentration of the NS layer was 5 % at a water contact angle (WCA) 110.5°. Further, Fourier transform infrared spectra showed the presence of SiOSi and SiCH3 groups in the NSS, NSC, and NSI films, and X-ray diffraction spectra indicated that original structures of these films were disordered. Moreover, the surface morphology, mechanical properties, and light transmission were affected by the NS layer, and the optimal layer was found to be NSI. After 10 days of storage at 100 % humidity, the NSI film exhibited low water vapor adsorption (37.22 g) and permeability (0.1484 g/m·s·Pa·10−11) and a high WCA (110.2°). In addition, the NSI film exhibited a visible color shift with an increasing pH of the buffer solution. A monitoring test of fish freshness showed that the NSI film displayed a distinctive color change corresponding to fish spoilage during 14 days of storage. This indicates that NSI has high potential in indicator film applications.

CO2 gas-triggered wettability control of silylation-modified CNC films by manipulating the surface structure and introducing tertiary amino groups

Here, cellulose nanocrystal (CNC) films were chemically modified in a two-stage process to realize surface wettability control through the introduction of CO2 gas. In addition to controlling the surface structure of the silylation-modified CNC film, functional groups derived from silane compounds were installed, and the corresponding effects on the resulting chemical modification were investigated. In the first stage, methyltriethoxysilane (MTES) and hexyltriethoxysilane (HTES) combined with tetraethoxysilane (TEOS) were subjected to condensation under alkaline conditions. In the second stage, (3-(N,N-dimethylamino)propyl)trimethoxysilane (DMAPS) generated an amino group to control the surface wettability by adsorption CO2 gas. Then, the silylation-modified CNC film was fabricated on a glass substrate by spin coating. Fourier transform infrared (FT-IR), nuclear magnetic resonance (29Si-NMR), and X-ray photoelectron spectroscopy (XPS) inspection indicated that the silane compounds were bonded to the CNC film surface and that tertiary amino groups were successfully introduced. The surface structure of the silylation-modified CNC film was analyzed by atomic force microscopy (AFM), and the surface roughness calculating indicated a root-mean-square roughness (RMS) of 4.2 nm. The water contact angles before and after the CO2 gas treatment were evaluated as 73o and 22o, respectively.

Pressure‐Induced Inversion of Chiroptical Properties in Cholesteric Assembled Cellulose Nanocrystals

AbstractDynamic manipulation of the chiroptical effect, particularly the inversion of chiral signal, has attracted broad attention, but still remains challenging, Here, stepwise tuning of the helicoidal assembly and chiroptical properties of the cellulose nanocrystal (CNC) films utilizing high pressure are demonstrated. With in situ monitoring, the changes of Fourier‐transform infrared (FTIR) spectra and evaluating the corresponding applied energy, three distinctive stages of chiroptical properties are clearly identified. In these stages, the photonic bandgap of the CNC film exhibits a fast‐slow–fast alteration over different energy imposed. The three stages are associated sequentially with interlayer space reduction, hydrogen bonding reconfiguration, and covalent bands changing. A switch of chiral signal from left‐ to right‐handedness is observed in the third stage, and the handedness of CD spectra can be recovered upon release, which is barely observed. This unique property is suspected to be the change in CNCs assembly induced by molecular‐level conformational change. This work sheds light on the fundamental understanding of how pressure can trigger the change from molecular‐level to hierarchical assembly and corresponding chiroptical performance, facilitating the design of chiral assembly with pre‐programmed triggering chiroptical properties of bio‐photonic materials.

Dark‐Bright Exciton Splitting Dominates Low‐Temperature Diffusion in Halide Perovskite Nanocrystal Assemblies

AbstractSemiconductor nanocrystals can replace conventional bulk materials completely in displays and light‐emitting diodes. Exciton transport dominates over charge carrier transport for materials with high exciton binding energies and long ligands, such as halide perovskite nanocrystal films. Here, how beneficial superlattices – nearly perfect 3D assemblies of nanocrystals ‐ are to exciton transport is investigated. Surprisingly, the high degree of order is not as crucial as the individual nanocrystal size, which strongly influences the splitting of the excitonic manifold into bright and dark states. At very low temperatures, the energetic splitting is larger for the smallest nanocrystals, and dark levels with low oscillator strength effectively trap excitons inside individual nanocrystals, suppressing diffusion. The effect is reversed at elevated temperatures, and the larger nanocrystal size becomes detrimental to exciton transport due to enhanced exciton trapping and dissociation. The results reveal that the nanocrystal size must be strongly accounted for in design strategies of future optoelectronic applications.

Enhancement of light emission of a low-resistivity silicon nanocrystal thin film: A simulational and experimental study

A silicon nanocrystal (Si NC) white light-emitting (λ = ∼400–900 nm) thin film with a relatively low resistivity of 1.6 × 104 Ω m has been prepared as an active medium for electrically driven Si light sources. The average size of Si NC is 2.4 ± 0.4 nm. To enhance the light emission efficiency of this low-resistivity Si NC thin film, approach of hydrogen passivation suitable for the traditional high-resistivity (1.2 × 107 Ω m in this work for example) red light-emitting Si NC thin film has been tried and found unavailable unfortunately. Our first principles simulation shows that Si NCs bonded to −O, −NH2, −OH, and −H ligands are responsible for red, green, and blue (RGB) primary color emissions in this white light-emitting sample, respectively. Passivation of the sample in NH3 and H2O atmosphere is then conducted, aiming to increase the number of the RGB light emitters. The light emission is significantly enhanced, with photoluminescence intensity, photoluminescence quantum yield, electroluminescence intensity, and net optical gains increased by factors of 4.6, 4.2, 4.0, and ∼3.0, respectively, after 10-day passivation. Further enhancements are expected for longer passivation.

Enhancement of the Silicon Nanocrystals’ Electronic Structure within a Silicon Carbide Matrix

Using plasma-enhanced chemical vapor deposition (PECVD), a mixed gas of silane (SiH4) and methane (CH4) was diluted with hydrogen (H2) to produce thin films of silicon nanocrystals embedded in a silicon carbide (SiC) matrix. This method prevents the co-deposition of SiH and SiC from high-temperature annealing procedures. This study experimentally explores the improvement of the electronic structure by adjusting two processing parameters according to classical nucleation theory (ratio of SiH4 to CH4 and working gas pressure). The deposited films were examined using ellipsometry spectroscopy, X-ray diffraction, scanning electron microscopy, atomic force microscopy, and photoluminescence to determine grain size, crystal volume fraction, topography, and bond configurations. The results show that increasing the working gas pressure can increase the density of SiC, while increasing the ratio of SiH4 to CH4 can only produce larger grain sizes. This is consistent with how SiC works and grows. Without using a high-temperature annealing procedure, this technique can improve the electrical structure of SiC contained in the SiC matrix formed by PECVD.

Nanoimaging of Water Bending and Carboxylate Stretching Modes in Colloidal Nanocrystal Films

Nanoimaging of Water Bending and Carboxylate Stretching Modes in Colloidal Nanocrystal Films

Faceted gold nanocrystal film over nanosphere for sensitive SERS distinction of fungicides by deep learning analysis

Metal film over nanosphere (MFON) as a classical SERS-active substrate has attracted numerous attention and applications due to its excellent sensitivity and signal uniformity. However, such MFON substrates prepared by metal deposition require strictly controlled experimental conditions with high cost to obtain high batch reproducibility. In addition, the precise adjustment of the metal particle fineness and crystal facet is hard to conduct, limiting the SERS signal for further improvement. In this work, we propose multifaceted Au nanocrystal over nanosphere (AuNCFON) structures based on a facile and robust interfacial assembly technique. Notably, the preparation method could reduce significant loss of metallic materials without the requirement of large-scale instrumentation, and allows precise control of the metal film thickness and the nanocrystal facet. Specifically, the size and facet of nanocrystals on PS spheres are systematically regulated and the Au dodecahedral films with {110} facet on PS arrays exhibit optimal SERS activity. The SERS intensity is amplified by an enhancement factor of 0.13 × 107 with excellent signal homogeneity and batch reproducibility. Finally, the sensitive and accurate distinction of similarly structured molecules including crystal violet and malachite green (fungicides) is demonstrated assisted by principal component analysis-partial least squares (PCA-PLS) algorithm. The AuNCFON SERS substrate combined with deep learning analysis has a broad application prospect in reliable identification of chemical pollutants.

Gallium oxide nanocrystals for self-powered deep ultraviolet photodetectors

Zero-dimensional colloidal nanocrystals (NCs) of gamma-phased gallium oxide (γ-Ga2O3) were successfully synthesized using the sol-gel method, resulting in nanocrystals with high crystallinity. Heterojunction photodetectors were then constructed by employing spin-coating technology to deposit γ-Ga2O3 NCs film of varying thicknesses onto p-type GaN substrates. The resulting devices demonstrated self-power capability through a photovoltaic effect when exposed to ultraviolet light illumination. Notably, a device with a 300 nm thick active layer, annealed in 400 °C, exhibited a responsivity of 6.7 × 10–3 A W–1, a detectivity of 3.10 × 1011 Jones, and an external quantum efficiency of 3.2 % under 254 nm light illumination at 0.16 mW cm–2, all without the need for an external power supply. These findings suggest promising practical applications for such photodetectors in single-point imaging systems. This study presents a straightforward and viable approach for developing high-performance and self-powered ultraviolet photodetectors based on zero-dimensional γ-Ga2O3 NCs, thereby opening up possibilities for various photonic systems and applications that do not rely on an external power supply.

Effect of Thickness on Structural, Morphological, and Optical Properties for Nanocrystalline Thin Films of Cd1-xSnxS, for Optoelectronic Applications

Thin films of Sn1-xCdxS nanocrystals (X=0,0.7, and 1) with different thicknesses 200, 300, and 400 nm were prepared on glass bases at 300 C by the participation method and chemical spray pyrolysis method (CSP). The effect of the concentration of Sn and thickness on the structural and optical properties of the prepared films was studied. The films were characterized to evaluate the structure, transmittance, and optical energy band gap. X-ray diffraction (XRD) studies showed that the films were polycrystalline with hexagonal and orthogonal structures. XRD and atomic force measurement results showed that the grain size increases with its thickness. The optical band gap energies and optical transmission and absorption types of the membranes were determined from the optical transmittance spectra. Optical absorption studies in the wavelength range 200-1100 nm showed that the energy gap values of the films decrease from 2.48-1.95 eV, 2.41-1.92 eV, and 2.3- 1,89 eV with increasing Sn concentration and with increase the thickness 200, 300, and 400 nm respectively. The best properties of the prepared material were determined for its possible use in the manufacture of solar cells.

Angular dependent terahertz emission from the interplay between nanocrystal diamond film and plasmonic metasurface

Composite structures integrated with metasurfaces and nonlinear films have emerged as alternative candidates to enhance nonlinear response. The cooperative interaction between the two components is complicated. Herein, a split-ring resonator (SRR)-type metasurface was fabricated on a free-standing nanocrystal diamond (NCD) film utilizing electron beam lithography, electron beam evaporation, and a lift-off process. The terahertz (THz) radiation from the SRR-NCD under normal incidence originates from the high-order magnetic resonance of SRR because the NCD film cannot produce detectable THz radiation at this incident angle. As increasing the incident angle, the contribution of the THz radiation from the NCD film gradually increases until reaching 40° incident angle limitation. The results indicate that this angular-dependent THz radiation is induced by the interplay between the NCD film and SRR. This study offers a new approach to investigate nonlinear processes in composite structures.

Enhancement of nonlinear optical effects of pure and Mn-doped CsPbBr3 perovskite films by external voltage

CsPbBr3 and Mn:CsPbBr3 nanocrystal (NC) films were prepared on indium tin oxide (ITO) glass by a slow solvent evaporation method. The ITO layer was applied a voltage to enhance the nonlinear optical (NLO) properties of CsPbBr3 and Mn:CsPbBr3 nanocrystal (NC) films. When the voltage is zero, the Z-scan results show that the nonlinear absorption (NLA) coefficients β of CsPbBr3 and Mn:CsPbBr3 NC films are 1.60 × 103 cm/GW and 1.54 × 104 cm/GW, respectively. The nonlinear refraction (NLR) coefficients γ of CsPbBr3 and Mn:CsPbBr3 NC films are 2.07 × 10−5 cm2/W and 2.48 × 10−5 cm2/W, respectively. Subsequently, the voltages of 5.03–13.42 V were applied to the ITO layer. Finally, when the voltage is 13.42 V, the β values of CsPbBr3 (1.75 × 104 cm/GW) and Mn:CsPbBr3 (3.98 × 104 cm/GW) NC films are 10.9 and 2.6 times higher than those without the applied voltage, respectively. The γ values of CsPbBr3 and Mn:CsPbBr3 NC films are 3.73 × 10−5 cm2/W and 3.88 × 10−5 cm2/W, respectively. This research proposes an effective method to improve the third-order NLO performances of CsPbBr3 and Mn:CsPbBr3 NC films by applying voltage.

Tunable gas selectivity of cellulose nanocrystals – Polyethylene glycol composite membranes

The kinetics of the CO2, D2 and CH4 transport through self-supporting cellulose nanocrystals (CNCs) films containing low-molecular weight poly(ethylene glycol) (PEG) additive as plasticizer agent was studied in single gas condition at 293 K and sub-atmospheric feed pressure. Neat CNC and PEG-CNC with 7 wt % PEG films act as impermeable barrier for CO2 and CH4 and exhibit D2 permeability values not larger than 0.05 barrer with penetrant diffusivity in the 10−10 cm2/s range. PEG addition at larger contents improves the permeability and diffusivity of all penetrant gases and gives to the PEG-CNC membrane a CO2 selective character. The CO2 permeability and ideal CO2/D2 selectivity of the PEG-CNC films are 0.42 ± 0.04 barrer and ∼3, respectively, at 14 wt % PEG content, 2.5 ± 0.2 barrer and ∼5 at 21 wt % PEG. The preferred CO2 transport exhibits a solution-selective character being the D2 diffusivity a factor ∼10 larger than the CO2 diffusivity. PEG addition, on the contrary, does not change the D2 selective character for the D2/CH4 gas couple. PEG well mixed with the gas-impermeable CNCs increases the PEG-CNC free volume and preferentially improves the CO2 solubility given the specific interactions between the quadrupolar CO2 penetrant and the dipolar ether groups of the PEG additive.

Facile Ligand Exchange of Ionic Ligand-Capped Amphiphilic Ag2S Nanocrystals for High Conductive Thin Films.

A surface ligand modification of colloidal nanocrystals (NCs) is one of the crucial issues for their practical applications because of the highly insulating nature of native long-chain ligands. Herein, we present straightforward methods for phase transfer and ligand exchange of amphiphilic Ag2S NCs and the fabrication of highly conductive films. S-terminated Ag2S (S-Ag2S) NCs are capped with ionic octylammonium (OctAH+) ligands to compensate for surface anionic charge, S2-, of the NC core. An injection of polar solvent, formamide (FA), into S-Ag2S NCs dispersed in toluene leads to an additional envelopment of the charged S-Ag2S NC core by FA due to electrostatic stabilization, which allows its amphiphilic nature and results in a rapid and effective phase transfer without any ligand addition. Because the solvation by FA involves a dissociation equilibrium of the ionic OctAH+ ligands, controlling a concentration of OctAH+ enables this phase transfer to show reversibility. This underlying chemistry allows S-Ag2S NCs in FA to exhibit a complete ligand exchange to Na+ ligands. The S-Ag2S NCs with Na+ ligands show a close interparticle distance and compatibility for uniformly deposited thin films by a simple spin-coating method. In photoelectrochemical measurements with stacked Ag2S NCs on ITO electrodes, a 3-fold enhanced current response was observed for the ligand passivation of Na+ compared to OctAH+, indicating a significantly enhanced charge transport in the Ag2S NC film by a drastically reduced interparticle distance due to the Na+ ligands.

Switchable circular polarized phosphorescence enabled by cholesteric assembled nanocelluloses

Circularly polarized room temperature phosphorescence (CP-RTP) has recently attracted great attention and is rapidly developing. To boost its potential applications in imaging, information encryption and optical detectors, high glum-factor and programmable chiroptical performance are needed. Herein, CP-RTP active materials were successfully constructed by infiltrating polycyclic aromatic hydrocarbon (naphthalene and pyrene) doped polymethyl methacrylate in cellulose nanocrystals (CNCs) films. It enabled a tunable glum-factor of up to −0.49 and an afterglow time of up to 8 s. Modulation of the matching degree of the photonic bandgap of CNCs films with the phosphor emission spectra allowed for achieving on-demand intensity, wavelength and handedness of CP-RTP and circularly polarized fluorescent (CP-FL) emission. Furthermore, dynamic tuning of the CP-RTP and CP-FL, including on/off, and positive/negative signal switching, was achieved through oxygen consumption by UV irradiation to reduce the effect of triplet oxygen quenching. The hybrid CNCs film demonstrated structural color, phosphorescence, and CP-RTP response, making it suitable for multiple channel information storage for anti-counterfeiting as a porotype. These findings show a range of prospective photonic applications, including information security, stereoscopic displays, and chiral polarizers.

Probing compositional engineering effects on lead-free perovskite-inspired nanocrystal thin films using correlative nonlinear optical microscopy.

We introduce the use of correlative third-harmonic generation and multiphoton-induced luminescence microscopy to investigate the impact of manganese (Mn) doping on bismuth (Bi)-based perovskite-inspired nanocrystal thin films. The technique was found to be extremely sensitive to the microscopic features of the perovskite film and its structural compositions, allowing the unambiguous detection of compositionally different emitters in the perovskite film and manipulation of their nonlinear optical responses. Our work unveils a new way to investigate, manipulate, and exploit perovskite-inspired functional materials for nonlinear optical conversion at the nanoscale.