Single Nanodots Research Articles

Heteronanotrimers by Selective Photodeposition of Gold Nanodots on Janus-Type Cu2‒ xS/CuInS2 Heteronanocrystals.

This study focuses on the development of environmentally friendly Au-Cu2-xS/CuInS2 heteronanotrimers. The chosen strategy relies on the laser photodeposition of a single gold nanodot (ND) onto Janus Cu2- xS/CuInS2 heteronanocrystals (HNCs). This method offers precise control over the number, location, and size (5 to 8nm) of the Au NDs by adjusting laser power for the career production, concentration of hole scavenger for charge equilibration in redox reactions, and gold precursor concentration, and exposure time for the final ND size. The photoreduction of gold ions onto HNCs starts systematically at the Cu2- xS tip. The Au deposition then depends on the CuInS2 segment length. For short HNCs, stable Au-Cu2- xS/CuInS2 heteronanotrimers form, while long HNCs undergo a secondary photo-induced step: the initial Au ND is progressively oxidized, with concomitant deposition of a second gold ND on the CuInS2 side, to yield Au2S-Cu2- xS/CuInS2-Au heteronanotrimers. Results are rationalized by quantitative comparison with a model that describes the growth kinetics of NDs and Au-Cu2- xS transformation and emphasizes the importance of charge separation in predicting selective deposition in heteronanotrimer production. The key parameter controlling Au-Cu2‒ xS/CuInS2 HNCs is the photoinduced electric field gradient generated by charge separation, which is tailored by controlling the CuInS2 segment size.

High quantum efficiency of hydrogen production from methanol aqueous solution with PtCu-TiO2 photocatalysts.

Methanol with 12.5 wt% H2 content is widely considered a liquid hydrogen medium. Taking into account water with 11.1 wt% H2 content, H2 synthesis from the mixture of water and methanol is a promising method for on-demand hydrogen production. We demonstrate an atomic-level catalyst design strategy using the synergy between single atoms and nanodots for H2 production. The PtCu-TiO2 sandwich photocatalyst achieves a remarkable H2 formation rate (2,383.9 µmol h-1) with a high apparent quantum efficiency (99.2%). Furthermore, the oxidation product is a high-value chemical formaldehyde with 98.6% selectivity instead of CO2, leading to a nearly zero-carbon-emission process. Detailed investigations indicate a dual role of the copper atoms: an electron acceptor to facilitate photoelectron transfer to Pt, and a hole acceptor for the selective oxidation of methanol to formaldehyde, thus avoiding over-oxidation to CO2. The synergy between Pt nanodots and Cu single atoms together reduces the activation energy of this process to 13.2 kJ mol-1.

CMOS-compatible manufacturability of sub-15 nm Si/SiO2/Si nanopillars containing single Si nanodots for single electron transistor applications

This study addresses the complementary metal-oxide-semiconductor-compatible fabrication of vertically stacked Si/SiO2/Si nanopillars (NPs) with embedded Si nanodots (NDs) as key functional elements of a quantum-dot-based, gate-all-around single-electron transistor (SET) operating at room temperature. The main geometrical parameters of the NPs and NDs were deduced from SET device simulations using the nextnano++ program package. The basic concept for single silicon ND formation within a confined oxide volume was deduced from Monte-Carlo simulations of ion-beam mixing and SiO x phase separation. A process flow was developed and experimentally implemented by combining bottom-up (Si ND self-assembly) and top-down (ion-beam mixing, electron-beam lithography, reactive ion etching) technologies, fully satisfying process requirements of future 3D device architectures. The theoretically predicted self-assembly of a single Si ND via phase separation within a confined SiO x disc of <500 nm3 volume was experimentally validated. This work describes in detail the optimization of conditions required for NP/ND formation, such as the oxide thickness, energy and fluence of ion-beam mixing, thermal budget for phase separation and parameters of reactive ion beam etching. Low-temperature plasma oxidation was used to further reduce NP diameter and for gate oxide fabrication whilst preserving the pre-existing NDs. The influence of critical dimension variability on the SET functionality and options to reduce such deviations are discussed. We finally demonstrate the reliable formation of Si quantum dots with diameters of less than 3 nm in the oxide layer of a stacked Si/SiO2/Si NP of 10 nm diameter, with tunnelling distances of about 1 nm between the Si ND and the neighboured Si regions forming drain and source of the SET.

Magnetization dynamics of single and trilayer permalloy nanodots

We have investigated the magnetization dynamics in single and trilayer circular permalloy nanodots with a diameter of 120 nm using broadband ferromagnetic resonance spectroscopy. For single-layer nanodots, two well-separated modes near the saturation field, a high-frequency center mode due to excitations at the center of the nanodots and a low-frequency edge mode due to the inhomogeneous effective field near the edges, were observed. Both the center mode and the edge mode are found to be sensitive to the thickness of the nanodots. However, for trilayer nanodots, two center modes arise due to the in-phase and out-of-phase precession of spins in magneto-dynamically coupled layers. Our experimental results are substantiated by micromagnetic simulations, which are in good agreement.

Deformations related to atom mixing in Si/SiO2/Si nanopillars under high-fluence broad-beam irradiation

Structures consisting of a single Si nanodot buried within an insulating nanometric ${\mathrm{SiO}}_{2}$ layer stacked between two Si layers show promising properties for room temperature operational single-electron transistors. Moreover, such structures are highly compatible with modern complementary metal-oxide semiconductor technologies. Metastable ${\mathrm{SiO}}_{\mathrm{x}}$ phase separates into a Si nanodot and insulating, homogeneous ${\mathrm{SiO}}_{2}$ during annealing, providing a solid path towards the desired structure. However, achieving the necessary amount of excessive Si, dissolved in the ${\mathrm{SiO}}_{2}$ for correct concentrations of ${\mathrm{SiO}}_{\mathrm{x}}$, remains a technological challenge. In this work, we investigate ion-induced atom mixing in pre-built $\mathrm{Si}/{\mathrm{SiO}}_{2}/\mathrm{Si}$ nanopillars, which is considered to be a technologically promising way to produce the necessary concentrations of spatially confined ${\mathrm{SiO}}_{\mathrm{x}}$ in a controlled manner. During the high-fluence ion irradiation, we notice a significant shortening of the nanopillar and preferential loss of O atoms. Both sputtering and nanoscale ion hammering are found to be the cause of the deformation. The ion-hammering effect on nanoscale is explained by multiple small displacements, strongly enhanced after the nanopillar was rendered completely amorphous. The methods presented here can be used to determine the ion-fluence threshold for sufficient atom mixing in spatially confined regions before the large structural deformations are formed.

Design of Metal@Titanium Oxide Nano-heterodimers by Laser-Driven Photodeposition: Growth Mechanism and Modeling.

In order to circumvent the usual nucleation of randomly distributed tiny metallic dots photodeposited on TiO2 nanoparticles (NPs) induced by conventional UV lamps, we propose to synthesize well-controlled nanoheterodimers (NHDs) using lasers focused inside microfluidic reactors to strongly photoactivate redox reactions of active ions flowing along with nanoparticles in water solution. Since the flux of photons issued from a focused laser may be orders of magnitude higher than that reachable with classical lamps, the production of electron-hole pairs is tremendously increased, ensuring a large availability of carriers for the deposition and favoring the growth of a single metallic dot as compared to secondary nucleation events. We show that the growth of single silver or gold nanodots can be controlled by varying the beam intensity, the concentration of the metallic salt, and the flow velocity inside the microreactor. The confrontation to a build-in model of the metallic nanodot light-induced growth onto the surface of TiO2 NPs shows the emergence of a predictable "master behavior" on which individual growths obtained from various tested conditions do collapse. We also characterized the associated quantum yield. Eventually, we successfully confronted our model to growth data from the literature in the case of silver on TiO2 and gold on II-VI semiconducting NPs triggered by UV lamps. It shows that for the photosynthesis of NHDs the efficiency of the electron-hole pair production rate matters much more than the number of pairs produced and that the use of laser light can provide a photodeposition-based synthesis at the nanoscale.

Influence of clustering round magnetic nano-dots on magnetization reversal

Square and round magnetic nano-dots of varying dimensions exhibit a large amount of possible magnetization reversal processes, from domain wall nucleation and propagation to multi-vortex states. Clustering such single nano-dots, however, may strongly modify these magnetization reversal processes due to the interactions between neighboring particles. Here we thus investigate the difference between magnetization reversal processes in clusters of hexagonally arranged round nano-dots under different orientations of the external magnetic field in comparison with single particle behavior.

In Situ Generated Silver Nanodot Förster Resonance Energy Transfer Pair Reveals Nanocage Sizes

Characterizing nanocages in macromolecules is one of the keys to understanding various biological activities and further utilizing nanocages for novel materials synthesis. However, fast and straightforward detection of the nanocage size remains challenging. Here, we present a new approach to detect the diameter of a nanocage by Förster resonance energy transfer (FRET) of luminescent silver nanodot pairs with reverse micelles as a model. Silver nanodot FRET pairs can be generated in situ from a single silver nanodot species with critical energy transfer distances, R0, of 4.8-6.5 nm. We have applied this approach to clarify the size variation of the water nanocage in nonionic surfactant Triton X-100-based reverse micelles. FRET efficiency decreases as more water is added, indicating that the size of the reverse micelles continuously expands with water content. The silver element in the nanocage also enhances the visualization of the nanocage under cryo-TEM imaging. The diameter of the water nanocage measured with the above approach is consistent with that obtained by cryo-TEM, demonstrating that the FRET measurement of silver nanodots can be a fast and accurate tool to detect nanocage dimensions. The above demonstration allows us to apply our strategy to other protein-based nanocages.

Nanodot array deposition via single shot laser interference pattern using laser-induced forward transfer

Laser-induced forward transfer (LIFT) is a direct-writing technique capable of depositing a single dot smaller than the laser wavelength at small shot energy through the laser-induced dot transfer (LIDT) technique. To deposit a single nanodot in a single shot of laser irradiation, a liquid nanodrop is transferred from donor to receiver and finally solidified via a solid–liquid–solid (SLS) process. In conventional LIDT experiments, multi-shots with step scanning have been used to form array structures. However, interference laser processing can achieve an arrayed process and generate a periodic structure in a single shot. In this study, a femtosecond laser interference pattern was first applied to LIDT, and an array of nanodots was successfully deposited in a single shot, producing the following unit structures: a single dot, adjoining dots, and stacking dots. The diameter of the smallest nanodot was 355 nm, and the narrowest gap between two adjoining nanodots was 17.2 nm. The LIDT technique produces high-purity, catalyst-free that do not require post-cleaning or alignment processes. Given these significant advantages, LIDT can expand the usability of nanodots in a wide range of fields.

Local ferroelectric studies on interconnected PVDF nano-dot thin films using piezo force microscopy

The local ferroelectric domain structure of interconnected PVDF nano-dots with applied DC voltages of ± 10 V to ± 100 V is evaluated using various Piezo force microscopy (PFM) methods such as conventional PFM and Dynamic contact electrostatic force microscopy (DC-EFM). The self-assembled PVDF nano-dots are fabricated by using a spin coater method with 2 wt% of PVDF-DMSO solution. The polar gamma phase is found to be induced by the microstructural confinement of PVDF monomers and also reveals a strong ferroelectric nature after poling at different DC bias voltages. Poling with altered directions of the voltages renders a 180° phase reversal of PVDF domains. The local domain study depicts that each nano-dot is functioning as a single domain and switches as a whole. The single nano-dot switching signifies the potential of using the nano-dots for organic ferroelectric memory devices.

Design, Preparation and Surface-enhanced Raman Scattering(SERS) Spectrum of Single Ag Nanodot†

Design, Preparation and Surface-enhanced Raman Scattering(SERS) Spectrum of Single Ag Nanodot†

Efficient Fabrication Process of Ordered Metal Nanodot Arrays for Infrared Plasmonic Sensor

In this paper, a simple process to fabricate ordered Au nanodot arrays up to 520 nm in diameter that respond to infrared light is developed, and the feasibility of its application to infrared plasmonic sensors is shown. The developed process utilizes thermal dewetting to agglomerate a coated gold film into nanodots. It was difficult to produce large nanodots that responded to infrared light owing to dot separation. In this paper, therefore, the mechanism of dot agglomeration by thermal dewetting is studied via an experiment and theoretical model, and conditions to form single nanodots are clarified. Furthermore, Au nanodot arrays of 100 nm to 520 nm in diameter were fabricated by this process, and their absorption spectra were analyzed. In addition, an analysis of the change in the peak wavelength against the refractive index indicates the possibility of further improvement of the sensitivity of the infrared plasmon sensors.

Effects of temperature and nitradition on phase transformation of GaN quantum dots grown by droplet epitaxy

Droplet epitaxy technique, the formation of gallium metallic droplets in the ultra-high vacuum and then nitridation with nitrogen plasma, was used to grow GaN nanodots on Si (111) by plasma-assisted molecular beam epitaxy. In this report, cubic GaN nanodots were successfully deposited on Si according to the characterizations of transmission electron microscopy (TEM), and we also investigate the influence of nitridation duration and substrate temperature on the surface roughness, crystal structure and density of GaN nanodots. The microstructures and phase transformation (from zinc-blende to wurtzite) of GaN nanodots are observed by controlling the irradiation time of nitrogen plasma (5, 7 and 10 min). Reflection high energy electron diffraction can in-situ monitor the process of droplet epitaxy. From the results of X-ray photoelectron spectroscopy, the sample grown at higher temperature and longer nitridation time had the strongest GaN bonding peak and the highest bonding ratio. That means GaN nanodots form more completely. The high-resolution TEM images of single GaN nanodot and selective area diffraction patterns of multi-GaN nanodots demonstrate the phase transformation of GaN nanodots from metastable zinc-blende to thermodynamically stable wurtzite structures as the nitridation time increases.

Ultrashort intrinsic-like channel FETs with nanodot-type floating gate utilizing biomaterial

We investigated the electrical characteristics of an n–i–n-type FET of a sub-10 nm channel with nanoparticles (NPs). To fabricate the FET, a V-groove was formed in a silicon-on-insulator substrate by anisotropic wet etching. Cage-shaped proteins (ferritin) was used for the formation and placement of NPs. A solution of ferritin containing a metal oxide core was dropped onto the substrate, and spin drying was performed to arrange the NPs at the bottom of the V-groove. NPs served as a floating gate for this device. Threshold voltage (Vth) shift was confirmed as a memory operation in devices with NPs. The Vth shift was clearly observed in different types of NPs. The Vth shift was controlled along the channel length direction by a single nanodot floating gate.

Influence of anisotropic dipolar interaction on the spin dynamics of Ni80Fe20 nanodot arrays arranged in honeycomb and octagonal lattices

Ultrafast spin dynamics in ferromagnetic nanodot arrays with dot diameter 100 nm and thickness 20 nm arranged in honeycomb and octagonal lattice symmetries are studied to explore the tunability of the collective magnetization dynamics. By varying the inter-dot separation between 30 nm and 300 nm drastic variation in the precessional dynamics from strongly collective to completely isolated regime has been observed by using all-optical time-resolved magneto-optical Kerr microscope. Micromagnetic simulation is exploited to gain insights about the resonant mode profiles and magnetic coupling between the nanodots. A significant spectral and spatial variation in the resonant mode with increasing dipolar interaction is demonstrated with increasing inter-dot separation. The spins driven by effective field inside single nanodots are prone to precess independently, generating two self-standing centre and edge modes in the array that are influenced by the relative orientation between the inter-dot coupling direction and bias magnetic field. The anisotropic behavior of dipolar field is rigorously investigated here. Splitting of the centre mode in case of octagonal lattice is experimentally observed here as a consequence of the anisotropic dipolar field between the nanodot pairs coupled horizontally and vertically, which is not found in the honeycomb lattice. In addition, proper understanding of the modification of dynamic mode profile by neighboring dipolar interaction built up here, is imperative for further control of the dynamic dipolar interaction and the corresponding collective excitation in magnonic crystals. The usage of nanodot lattices with complex basis structures can be advantageous for the designing of high density magnetic recording media, spin-wave filter and logic devices.

Charge-Driven Fluorescence Blinking in Carbon Nanodots.

This study focuses on the mechanism of fluorescence blinking of single carbon nanodots, which is one of their key but less understood properties. The results of our single-particle fluorescence study show that the mechanism of carbon nanodots blinking has remarkable similarities with that of semiconductor quantum dots. In particular, the temporal behavior of carbon nanodot blinking follows a power law both at room and at cryogenic temperatures. Our experimental data suggest that static quenching via Dexter-type electron transfer between surface groups of a nanoparticle plays a major role in the transition of carbon nanodots to off or gray states, whereas the transition back to on states is governed by an electron tunneling from the particle's core. These findings advance our understanding of the complex mechanism of carbon nanodots emission, which is one of the key steps for their application in fluorescence imaging.

Investigation of magnetization dynamics in 2D Ni80Fe20 diatomic nanodot arrays

Magnetization dynamics in Ni80Fe20 (Py) diatomic nanodots (nanodots of the same thickness but with large and small diameters that are closely placed to each other so as to act as a diatomic basis structure) embedded in 2D arrays have been investigated by the Brillouin light scattering technique. A distinct variation of resonant mode characteristics for different in-plane bias magnetic field applied along two different orientations of the lattice has been observed. Micromagnetic simulations reproduced the observed dynamical behaviour and revealed the variation of spatial distribution of collective modes of constituent single nanodots with different diameter and a diatomic unit forming the large array to understand the evolution of the magnetization dynamics from a single dot to the large array via a diatomic unit. The changes in mode frequency, spatial profiles of the modes, and appearance of new modes in a diatomic unit and its array from that of the constituent single dots indicate the strong magnetostatic interaction among the dots within the diatomic unit. Also, the occurrence of the new interacting mode at different frequencies for different orientations of the bias field indicates the change in the nature of interaction among the dots within the diatomic unit with bias magnetic field. The mode profiles also show distinct behaviour for smooth and rough-edged dots. This work motivates the study of magnonic band structure formation of such a dipolarly coupled nanodot array containing a complex double-dot unit cell.

Size dependence of switching behavior in single epitaxial Co/Pt multilayer nanodots

We have studied the magnetization switching behavior in single nanodots of an epitaxially grown Co/Pt multilayer with various dot diameters (d) ranging from 120 to 1600 nm. The switching behaviors in all the dots used in this study are considered to be of the nucleation type from the angular dependence of the switching field. However, detailed analysis of the thermally activated magnetization switching reveals that there is a critical diameter (dc) of 400 nm above which the switching process changes. For d < dc, the switching behavior of the dot is well explained by the nucleation process based on the single-energy barrier model. On the other hand, the switching behavior for d ≥ dc obviously deviates from that for d < dc and can be plausibly explained by the multi-barrier model.

Magnetization reversal using excitation of collective modes in nanodot matrices.

The large arrays of magnetic dots are the building blocks of magnonic crystals and the emerging bit patterned media for future recording technology. In order to fully utilize the functionalities of high density magnetic nanodots, a method for the selective reversal of a single nanodot in a matrix of dots is desired. We have proposed a method for magnetization reversal of a single nanodot with microwave excitation in a matrix of magneto-statically interacting dots. The method is based on the excitation of collective modes and the spatial anomaly in the microwave power absorption. We perform numerical simulations to demonstrate the possibility of switching a single dot from any initial state of a 3 by 3 matrix of dots, and develop a theoretical model for the phenomena. We discuss the applicability of the proposed method for introducing defect modes in magnonic crystals as well as for future magnetic recording.

Microwave-assisted switching of a single perpendicular magnetic tunnel junction nanodot

We used a magnetic tunnel junction (MTJ) to study the microwave-assisted switching (MAS) of a perpendicular magnetic nanodot. Because the sample structure enables the direct observation of fast magnetization dynamics through MTJ signals, the switching behavior is examined in relation with the induced magnetization excitation. By utilizing the MTJ signals, we observe several ferromagnetic resonance (FMR) modes and identify the lowest FMR mode as the cause of switching field reduction. The switching field decreases as the microwave frequency increases, and this frequency–magnetic field relationship coincides with that of the lowest FMR mode near a critical frequency. We also investigate the size dependence of MAS.