Emerging Atomic Energy Levels in Zero-Dimensional Silicon Quantum Dots.

Driven by the emergence of colloidal semiconductor quantum dots (QDs) of tunable emission wavelengths, characteristic of exciton absorption peaks, outstanding photostability and solution processability in device fabrication have become a key tool in the development of nanomedicine and optoelectronics. Diamond cubic crystalline silicon (Si) QDs, with a diameter larger than 2 nm, terminated with hydrogen atoms are known to exhibit bulk-inherited spin and valley properties. Herein, we demonstrate a newly discovered size region of Si QDs, in which a fast radiative recombination on the order of hundreds of picoseconds is responsible for photoluminescence (PL). Despite retaining a crystallographic structure like the bulk, controlling their diameters in the 1.1-1.7 nm range realizes the strong PL with continuous spectral tunability in the 530-580 nm window, the narrow spectral line widths without emission tails, and the fast relaxation of photogenerated carriers. In contrast, QDs with diameters greater than 1.8 nm display the decay times on the microsecond order as well as the previous Si QDs. In addition to the five-orders-of-magnitude variation in the PL decay time, a systematic study on the temperature dependence of PL properties suggests that the energy structure of the smaller QDs does not retain an indirect band gap character. It is discussed that a 1.7 nm diameter is critical to undergo changes in energy structure from bulky to molecular configurations.

Selective Detection of Target Volatile Organic Compounds in Contaminated Air Using Sensor Array with Machine Learning: Aging Notes and Mold Smells in Simulated Automobile Interior Contaminant Gases.

We investigated the selective detection of target volatile organic compounds (VOCs) which are age-related body odors (namely, 2-nonenal, pelargonic acid, and diacetyl) and a fungal odor (namely, acetic acid) in the presence of interference VOCs from car interiors (namely, n-decane, and butyl acetate). We used eight semiconductive gas sensors as a sensor array; analyzing their signals using machine learning; principal-component analysis (PCA), and linear-discriminant analysis (LDA) as dimensionality-reduction methods; k-nearest-neighbor (kNN) classification to evaluate the accuracy of target-gas determination; and random forest and ReliefF feature selections to choose appropriate sensors from our sensor array. PCA and LDA scores from the sensor responses to each target gas with contaminant gases were generally within the area of each target gas; hence; discrimination between each target gas was nearly achieved. Random forest and ReliefF efficiently reduced the required number of sensors, and kNN verified the quality of target-gas discrimination by each sensor set.

Self-assembly patterning of ultrafine zirconia nanocrystal films fabricated on chemically patterned templates.

Ultra-thin zirconia (ZrO2) nanocrystal films were fabricated by using a controlled dip-coating process. ZrO2 nanocrystals possess a cubic crystalline phase and large surface-to-volume area. The film composite with only several layers of nanocrystals were obtained by controlling the withdrawal speed and mass concentration of the colloidal solution. The optical properties of ZrO2 nanocrystal films were accessed by UV-vis spectroscopy, which indicated the dense and uniform structure of the nanocrystal films. The high reflection index suggested that the films could be used in the reflection coating industry. Furthermore, a micro-pattern of self-assembled monolayers of silane molecular was used as a chemical mold for selective deposition of ZrO2 nanocrystals. As a result, a self-assembly patterning of ZrO2 nanocrystals with a neat edge was fabricated on silicon substrate. The low-cost fabricating method is compatible with conventional very-large-scale integration processes and can be extended to other kinds of nanocrystals.

Transition-Metal-Doped NIR-Emitting Silicon Nanocrystals.

Impurity-doping in nanocrystals significantly affects their electronic properties and diversifies their applications. Herein, we report the synthesis of transition metal (Mn, Ni, Co, Cu)-doped oleophilic silicon nanocrystals (SiNCs) through hydrolysis/polymerization of triethoxysilane with acidic aqueous metal salt solutions, followed by thermal disproportionation of the resulting gel into a doped-Si/SiO2 composite that, upon HF etching and hydrosilylation with 1-n-octadecene, produces free-standing octadecyl-capped doped SiNCs (diameter≈3 to 8 nm; dopant <0.2 atom %). Metal-doping triggers a red-shift of the SiNC photoluminescence (PL) of up to 270 nm, while maintaining high PL quantum yield (26 % for Co doping).

Halogen- and acid-free syntheses of TiO2 nanocrystal coatings and high surface area TiO2 nanocrystal-assembled particles.

Halogen- and acid-free aqueous solutions were developed to synthesize TiO2 nanocrystal coatings and high surface area TiO2 nanocrystal-assembled particles. Fluorine doped tin oxide substrates were covered with TiO2 nanocrystal coatings in the solutions containing titanium(IV) potassium oxalate, 2-hydrate (K2[TiO(C2O4)2] x 2H2O) at 90 degrees C for 3 days. The substrates changed color by the coatings. The nanocrystals were about 10-50 nm in size. They connected with the substrates strongly due to direct nucleation and crystallization on the substrates. Furthemore, TiO2 particles were synthesized from the solutions at 90 degrees C for 3 days. They were assemblies of TiO2 nanocrystals. Brunauer-Emmett-Teller surface area reached to 275 m2/g. Nanospaces were formed by assembly of the nanocrysrtals. They contributed high surface area. Halogen- and acid-free aqueous solution processes had advantages of low environmental load and safety manufacturing environment. Harmless TiO2 coatings and particles can be applied to medical applications, medicines, cosmetics, food additives, etc.

One dimensional spindle titanium oxide nanocrystals.

Anatase TiO2 nanocrystals of 10 nm were formed in aqueous solutions at 90 degrees C for 1 day. The solutions contained ammonium hexafluorotitanate (5 mM) and boric acid (15 mM). The nanocrystals connected each other to form particles of about 100-500 nm. X-ray diffraction, Raman and FT-IR analysis revealed characteristics of the nanocrystals. They had BET surface area of 68 m2/g. Furthermore, morphology was controlled to one dimensional spindle shape by change of supersaturation degree. The spindle nanocrystals of 30 nm in width and 100 nm in length were formed in low concentration solutions. They contained hexafluorotitanate of 0.5 mM and boric acid of 0.15 mM. The spindle nanocrystals had higher crystallinity compared to the above nanocrystals. BET surface area reached 88 m2/g. One dimensional structure was realized by anisotropic crystal growth along c-axis. Low supersaturation degree decreased growth speed and brought out anisotropic crystal growth.

Long term synthesis of needle crystal assembled TiO2 films in an aqueous solution.

Crystals of TiO2 grew on substrates in aqueous solutions at 50 degrees C for 180 day. Long term synthesis brought us nano-structured films and information of crystal growth. The films showed 101 diffraction peak of anatase phase. Morphology of the crystals well developed to needle shapes. They stood perpendicular to the substrates. The films had nano-structured surfaces. Surface nanostructures would contribute to high sensitive sensors, solar cells, photocatalysis, etc. The films had high transparency compared to bare substrates in the range 360-525 nm.

Water bath synthesis of tin oxide nanostructure coating for a molecular sensor.

Tin oxide nanostructures were fabricated using a water bath technique. The structures were modified with dye-labeled DNAs for a molecular sensor. Sensing mechanism of the sensor was based on a photoelectric conversion effect. Photoluminescence intensities from the tin oxide nanostructures reached to 16 times larger than that from SnO2:F films. High photocurrent of 5.5 x 10(-6) A and high signal-to-noise ratio of 29 were achieved in this system. Photoelectric conversion on a combination of the dye-labeled DNA and the tin oxide was an essence of the sensing system. Surface nanospaces were effectively utilized to increase photoluminescence and photocurrent.

Liquid phase deposited titania coating to enable in vitro apatite formation on Ti6Al4V alloy.

A recently developed "GRAPE(®) technology" provides titanium or titanium alloy implants with spontaneous apatite-forming ability in vitro, which requires properly designed gaps and optimum heat treatment in air. In this study, titanium alloy and commercially pure (cp) titanium substrates were thermally oxidized in air before aligning pairs of specimens in the GRAPE(®) set-up, i.e., titanium alloy and cp titanium substrates were aligned parallel to each other with optimum gap width (spatial design). A liquid phase deposition (LPD) technique was employed for titania coatings on titanium alloy substrate. Then, they were soaked in Kokubo's simulated body fluid (SBF, pH 7.4, 36.5 °C) for 7 days to confirm the in vitro apatite formation on the substrates under the specific spatial design. Anatase-type titania coatings fabricated by using LPD technique led to the deposition of apatite particles within 7 days and showed apatite X-ray diffraction. On the other hand, thermally oxidized titanium alloy substrate in air and non-treated specimens did not show any apatite X-ray diffraction. These results indicated that the heterogeneous nucleation of apatite induced on anatase-type titania coating prepared by LPD technique when it was aligned parallel to thermally oxidized cp titanium substrate with optimum gap width.

Size-dependent color tuning of efficiently luminescent germanium nanoparticles.

It is revealed that rigorous control of the size and surface of germanium nanoparticles allows fine color tuning of efficient fluorescence emission in the visible region. The spectral line widths of each emission were very narrow (<500 meV). Furthermore, the absolute fluorescence quantum yields of each emission were estimated to be 4-15%, which are high enough to be used as fluorescent labeling tags. In this study, a violet-light-emitting nanoparticle is demonstrated to be a new family of luminescent Ge. Such superior properties of fluorescence were observed from the fractions separated from one mother Ge nanoparticle sample by the fluorescent color using our developed combinatorial column technique. It is commonly believed that a broad spectral line width frequently observed from Ge nanoparticle appears because of an indirect band gap nature inherited even in nanostructures, but the present study argues that such a broad luminescence spectrum is expressed as an ensemble of different spectral lines and can be separated into the fractions emitting light in each wavelength region by the appropriate postsynthesis process.

Large-lateral-area SnO2 nanosheets with a loose structure for high-performance acetone sensor at the ppt level.

Gas sensors with high sensitivity and high selectivity are required in practical applications to distinguish between target molecules in the detection of volatile organic compounds, real-time security alerts, and clinical diagnostics. Semiconducting tin oxide (SnO2) is highly regarded as a gas-sensing material due to its exceptional responsiveness to changes in gaseous environments and outstanding chemical stability. Herein, we successfully synthesized a large-lateral-area SnO2 nanosheet with a loose structure as a gas sensing material by a one-step facile aqueous solution process without a surfactant or template. The SnO2 sensor exhibited a remarkable sensitivity (Ra/Rg = 1.33) at 40 ppt for acetone, with a theoretical limit of detection of 1.37 ppt, which is the lowest among metal oxide semiconductor-based gas sensors. The anti-interference ability of acetone was higher than those of pristine SnO2 and commercial sensors. These sensors also demonstrated perfect reproducibility and long-term stability of 100 days. The ultrasensitive response of the SnO2 nanosheets toward acetone was attributed to the specific loose large lateral area structure, small grain size, and metastable (101) crystal facets. Considering these advantages, SnO2 nanosheets with larger lateral area sensors have great potential for the detection and monitoring of acetone.

Synthesis and characterization of visible-light-driven novel CuTa2O6 as a promising practical photocatalyst.

In this work, the novel CuTa2O6 phase was successfully synthesized by the hydrothermal and followed by the calcination process. The X-ray diffraction pattern confirms the formation of different phases. At a low temperature, CuTa2O6 exhibits the orthorhombic phase, whereas, at a higher temperature, it underwent a phase transition to a cubic crystal structure. X-ray photoelectron spectroscopic results suggest the presence of all the elements (Cu, Ta, and O). The optical studies were carried out using a UV-Vis DRS spectrophotometer. FESEM images confirm the spherical-shaped particles for the sample annealed at a high temperature. The local atomic and electronic structures around Cu and the contribution of the Cu oxidation state in the CuTa2O6 system were determined by X-ray absorption spectroscopy. To investigate the effective usage of CuTa2O6 in treating wastewater, its photocatalytic activity was investigated by evaluating its use in the photodegradation of MO dye under visible light irradiation. Moreover, the prepared CuTa2O6 photocatalyst exhibits significant photocatalytic activity in the degradation of MO dye and shows excellent stability; it is therefore a promising material for potential use in a practical photocatalyst. The CuTa2O6 photocatalyst suggests an alternative avenue of research into effective photo-catalysts for solar hydrogen water splitting.

Novel GdTaO4 phase for efficient photocatalytic degradation of organic dye under visible light irradiation: An X-ray spectroscopic investigation.

The novel GdTaO4 phase exhibits good photocatalytic activity under visible light irradiation and holds great promise for the removal of organic dyes from industrial wastes. The GdTaO4 samples were synthesized using the hydrothermal and calcination process with different weight ratios of gadolinium nitrate hydrate (G) and tantalum pentachloride (T), and their structural studies confirmed the formation of the GdTaO4 (GT) phase. Among the samples, GT-4 (with a weight ratio of 4:1) exhibited the highest photocatalytic activity for the degradation of Methyl Orange (MO) dye under visible light irradiation. To enhance the photocatalytic performance, H2O2 was used as a green additive, and the photocatalytic abilities were examined by varying dye types and concentrations. X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) revealed the local atomic and electronic structures around Ta and Gd and highlighted the contribution of Gd3+ to the GT system, which is a crucial factor in supporting the enhanced photocatalytic performance. Moreover, in-situ XAS at Gd M5-edge and O K-edge were examined under illumination/dark conditions to explore the electronic structures of photo-excited electron transition in the photocatalytic process. The analytical results provided strong evidence correlating the electronic structure and photocatalytic property of the GT. This study demonstrates that GdTaO4 exhibits good photocatalytic activity under visible light irradiation, making it a promising new Ta-based photocatalyst for the effective removal of organic dyes from industrial wastes.

Nanosheet-type tin oxide gas sensor array for mental stress monitoring.

Mental stress management has become significantly important because excessive and sustained mental stress can damage human health. In recent years, various biomarkers associated with mental stress have been identified. One such biomarker is allyl mercaptan. A nanosheet-type tin oxide exhibited high gas selectivity for allyl mercaptan; thus, in this study, a sensor array comprising nanosheet-type tin oxide gas sensors was fabricated to detecting allyl mercaptan. Supervised learning algorithms were use to build gas classification models based on the principal component analysis of the sensor signal responses from the sensor array. The comprehensive data provided by the classification models can be used to forecast allyl mercaptan with high accuracy.

Facet-Controlled Synthesis of CeO2 Nanoparticles for High-Performance CeO2 Nanoparticle/SnO2 Nanosheet Hybrid Gas Sensors.

CeO2 nanocubes with metastable {100} facets and CeO2 nanooctahedrons with the most stable {111} facets are herein fabricated by controlling the morphology and facets of CeO2 nanoparticles. SnO2 nanosheet-based assembled films coated with these CeO2 nanocubes or CeO2 nanooctahedrons yield {100} CeO2 nanocubes/SnO2 nanosheets and {111} CeO2 nanooctahedron/SnO2 nanosheet hybrid gas sensors, respectively. The hybrid sensors with CeO2 nanoparticles exhibited enhanced sensing responses to numerous chemical species relative to a pristine SnO2 nanosheet gas sensor, including acetone, hydrogen, ethanol, ammonia, acetaldehyde, and allyl mercaptan. In particular, the responses of {100} CeO2 nanocubes/SnO2 nanosheets and {111} CeO2 nanooctahedron/SnO2 nanosheet gas sensors to acetone or allyl mercaptan were 6.8 and 10.3 times higher, respectively, than that of the pristine SnO2 nanosheet gas sensor. Furthermore, the sensor response to ammonia was 2.5 times higher than that of a commercial volatile organic compound (VOC) gas sensor (TGS2602, Figaro Engineering Inc.). The CeO2 nanocube-based sensor with exposed metastable {100} facets promotes the adsorption and oxidation of VOCs owing to the higher surface energy of the metastable {100} facets and therefore exhibits a higher sensing performance than the CeO2 nanooctahedron-based sensor with an exposed {111} facet. The developed sensors show excellent potential for the detection of gas markers in human breath and perspiration for disease diagnosis.

Highly Sensitive and Selective Gas Sensors Based on NiO/MnO2 @NiO Nanosheets to Detect Allyl Mercaptan Gas Released by Humans under Psychological Stress.

NiO nanosheets are synthesized in situ on gas sensor chips using a facile solvothermal method. These NiO nanosheets are then used as gas sensors to analyze allyl mercaptan (AM) gas, an exhaled biomarker of psychological stress. Additionally, MnO2 nanosheets are synthesized onto the surfaces of the NiO nanosheets to enhance the gas-sensing performance. The gas-sensing response of the NiO nanosheet sensor is higher than that of the MnO2 @NiO nanosheet sensor. The response value can reach 56.69, when the NiO nanosheet sensor detects 40 ppm AM gas. Interestingly, a faster response time (115 s) is obtained when the MnO2 @NiO nanosheet sensor is exposed to 40 ppm of AM gas. Moreover, the selectivity toward AM gas is about 17-37 times greater than those toward confounders. The mechanism of gas sensing and the factors contributing to the enhance gas response of the NiO and MnO2 @NiO nanosheets are discussed. The products of AM gas oxidized by the gas sensor are identified by gas chromatography-mass spectrometry (GC/MS). AM gas detection is an unprecedented application for semiconductor metal oxides. From a broader perspective, the developed sensors represent a new platform for the identification and monitoring of gases released by humans under psychological stress, which is increasing in modern life.

Examination of VOC Concentration of Aroma Essential Oils and Their Major VOCs Diffused in Room Air.

This study analyzed temporal variation of the composition of volatile organic compounds (VOCs) at different diffusion time of gaseous phase of aroma compounds of four essential oils, lavender, tea tree, eucalyptus, and melissa. GC/MS methodology with the trace gas sampling by a thermal desorption tube is used to quantitatively determine the concentration of the corresponding 14 kinds of major and original VOCs in four essential oils. This study revealed for the first time that the concentration level of gaseous phase composition is varied, with a diffusion time from that of the liquid phase at equilibrium with it and the VOCs in the essential oils are classified into two groups, depending on whether their concentration with the time. It is verified that the total concentration of VOCs of these essential oils in the room air diffused by the ultrasonic diffuser is as low as 0.6 ppb and decreased soon below 0.1 ppb.

Self-Adaptive Gas Sensor System Based on Operating Conditions Using Data Prediction.

Through the improvement of nanomaterial technologies, a gas sensor was developed for detecting ppm or ppb levels of gas. Our SnO2 nanosheet gas sensor can detect 50 ppb of acetone without the requirement of a novel metal catalyst by exposing the (101) facet containing the Sn2+ state. Despite the high performance, the fluctuation of the gas response value based on operating conditions, even at the same concentration, is a critical problem in gas sensors. Thus, the alarm criteria of the sensor are typically determined by a safety factor. However, this method is not suitable for application in ultrasensitive sensors that require distinguishing minute differences in extremely low concentrations for medical examination or odor analysis. Therefore, we suggest a self-adaptive system that is based on operating conditions in collaboration with the data prediction model. The sensor system is based on a predictive model obtained by the response surface methodology. When the system detects a change in conditions, the alarm criteria are changed appropriately through the calculated values from the predictive model. To prepare a database for an effective predictive model, the gas responses of the SnO2 nanosheet sensor were measured with 20 treatments with 3 independent variables, namely, the temperature, flow rate, and concentration. Our prediction model achieved its best performance on training data with R2 = 0.9299 and less than 5% error in the prediction of unseen data.

Corrosion resistance and durability of superhydrophobic surface formed on magnesium alloy coated with nanostructured cerium oxide film and fluoroalkylsilane molecules in corrosive NaCl aqueous solution.

The corrosion resistant performance and durability of the superhydrophobic surface on magnesium alloy coated with nanostructured cerium oxide film and fluoroalkylsilane molecules in corrosive NaCl aqueous solution were investigated using electrochemical and contact angle measurements. The durability of the superhydrophobic surface in corrosive 5 wt% NaCl aqueous solution was elucidated. The corrosion resistant performance of the superhydrophobic surface formed on magnesium alloy was estimated by electrochemical impedance spectroscopy (EIS) measurements. The EIS measurements and appropriate equivalent circuit models revealed that the superhydrophobic surface considerably improved the corrosion resistant performance of magnesium alloy AZ31. American Society for Testing and Materials (ASTM) standard D 3359-02 cross cut tape test was performed to investigate the adhesion of the superhydrophobic film to the magnesium alloy surface. The corrosion formation mechanism of the superhydrophobic surface formed on the magnesium alloy was also proposed.

Corrosion resistant performances of alkanoic and phosphonic acids derived self-assembled monolayers on magnesium alloy AZ31 by vapor-phase method.

Alkanoic and phosphonic acid derived self-assembled monolayers (SAMs) were formed on magnesium alloy by the vapor phase method. AFM and XPS studies showed that SAMs were formed on Mg alloy. The chemical and anticorrosive properties of the SAMs prepared on magnesium alloys were characterized using contact angle measurements, X-ray photoelectron spectroscopy (XPS), and electrochemical measurements. Water contact angle measurements revealed that, although SA and ISA have the same headgroup to anchor to the magnesium alloy surface, the packing density on the magnesium alloy surface could be considerably different. The contact angle hysteresis of SAMs with a carboxylate headgroup is much larger than that of SAMs with a phosphonic acid group. The XPS O 1s peaks indicated more likely a mix of mono-, bi-, or tridentate binding of phosphonic acid SAM to the oxide or hydroxide surface of the Mg alloy. The electrochemical measurements showed that the phosphonic acid derived SAM had better corrosion resistance compared to alkanoic acid derived SAM. The chemical stability of SAMs modified magnesium alloy was investigated using water contact angle and XPS measurements. The water contact angle and XPS measurements revealed that the molecular density of OP and PFEP on magnesium alloy would be higher than those of SA and ISA on magnesium alloy.