Nanodot Research Articles

High-density formation of Ta nanodot induced by remote hydrogen plasma

We have studied the formation of Ta nanodots (NDs) on thermally grown SiO2/Si by exposing a thin metal layer to a remote H2 plasma (H2-RP) without external heating. Atomic force microscopy (AFM) analyses show that a combination of a Ge (∼30.0 nm)/Ta (∼2.0 nm) bilayer stack with subsequent H2-RP exposure is effective for forming electrically isolated Ta NDs with an areal dot density as high as ∼1011 cm−2, where the Ge layer plays an important role as a barrier layer against the oxidation of the ultrathin Ta layer surface. The change in the chemical structure of the Ge/Ta bilayer stack on SiO2/Si upon Ta ND formation by H2-RP exposure is investigated by hard X-ray photoemission spectroscopy (HAXPES) and X-ray photoelectron spectroscopy (XPS).

Pentacene memory transistors with a monolayer of ligand‐removed semiconductor colloidal nanodots used as a charge‐trapping layer

We fabricated pentacene memory transistors of which floating gate or charge‐trapping layer consisted of monolayer of PbS colloidal nanodots (NDs), and investigated their writing and retention characteristics. Both of the writing and retention times of the pentacene memory transistors were significantly reduced when the ligand molecules (oleic acid) were removed. We also found that the writing time was further reduced and the retention time was extended by using larger NDs, in which the quantized energy level in the conduction band was lower, that is, closer to the highest occupied molecular orbital (HOMO) and farther from the lowest unoccupied molecular orbital (LUMO) of the pentacene. These results are explained with a model that the memory effect is due to the trapped electrons in the NDs after a positive writing voltage is applied on the control gate, and suggest that more suitable arrangement of the quantized energy level in the NDs and HOMO/LUMO levels in organic molecules would improve the characteristics of the memory transistors.

Nano spin-diodes using FePt-NDs with huge on/off current ratio at room temperature.

Spin transistors have attracted tremendous interest as new functional devices. However, few studies have investigated enhancements of the ON/OFF current ratio as a function of the electron spin behavior. Here, we found a significantly high spin-dependent current ratio—more than 102 at 1.5 V—when changing the relative direction of the magnetizations between FePt nanodots (NDs) and the CoPtCr-coated atomic force microscope (AFM) probe at room temperature. This means that ON and OFF states were achieved by switching the magnetization of the FePt NDs, which can be regarded as spin-diodes. The FePt magnetic NDs were fabricated by exposing a bi-layer metal stack to a remote H2 plasma (H2-RP) on ~1.7 nm SiO2/Si(100) substrates. The ultrathin bi-layers with a uniform surface coverage are changed drastically to NDs with an areal density as high as ~5 × 1011 cm−2. The FePt NDs exhibit a large perpendicular anisotropy with an out-of-plane coercivity of ~4.8 kOe, reflecting the magneto-crystalline anisotropy of (001) oriented L10 phase FePt. We also designed and fabricated double-stacked FePt-NDs with low and high coercivities sandwiched between an ultra-thin Si-oxide interlayer, and confirmed a high ON/OFF current ratio when switching the relative magnetization directions of the low and high coercivity FePt NDs.

NO gas sensing at room temperature using single titanium oxide nanodot sensors created by atomic force microscopy nanolithography

In this work, the fabrication of single titanium oxide nanodot (ND) resistive sensors for NO gas sensing at room temperature is reported. Two atomic force microscopy nanolithography methods, nanomachining and nano-oxidation, are employed. A single titanium nanowire (NW) is created first along with contact electrodes and a single titanium oxide ND is subsequently produced in the NW. Gas sensing is realized by the photo-activation and the photo-recovery approaches. It is found that a sensor with a smaller ND has better performance than a larger one. A response of 31%, a response time of 91 s, and a recovery time of 184 s have been achieved at a concentration of 10 ppm for a ND with a size of around 80 nm. The present work demonstrates the potential application of single metal oxide NDs for gas sensing with a performance that is comparable with that of metal oxide nanowire gas sensors.

Impact of embedded Mn nanodots on resistive switching characteristics of Si-rich oxides as measured in Ni-electrode metal–insulator–metal diodes

We studied the formation of high-density Mn nanodots (NDs) by remote H2 plasma (H2-RP) treatment and investigated how the embedding of Mn NDs affects the resistive switching properties of Si-rich oxides (SiOx) because it is expected that NDs will trigger the formation of the conductive filament path in SiOx. We fabricated Mn NDs with different ND areal densities by exposing an ultrathin Mn layer on the SiOx/Ni bottom electrode to H2-RP without external heating. After the deposition of SiOx as a resistive switching layer and the fabrication of Ni top electrodes, resistive switching behaviors of metal–insulator–metal (MIM) diodes were characterized from current–voltage (I–V) curves and compared with the results obtained from samples of SiOx without Mn NDs and with an ultrathin Mn layer instead. The embedding of Mn NDs in SiOx was found to be effective in increasing the ON/OFF ratio in resistance and reducing the variation in operation voltage.

Rapid Degradation and High Renal Clearance of Cu3BiS3 Nanodots for Efficient Cancer Diagnosis and Photothermal Therapy in Vivo.

A key challenge for the use of inorganic nanomedicines in clinical applications is their long-term accumulation in internal organs, which raises the common concern of the risk of adverse effects and inflammatory responses. It is thus necessary to rationally design inorganic nanomaterials with proper accumulation and clearance mechanism in vivo. Herein, we prepared ultrasmall Cu3BiS3 nanodots (NDs) as a single-phased ternary bimetal sulfide for photothermal cancer therapy guided by multispectral optoacoustic tomography (MSOT) and X-ray computed tomography (CT) due to bismuth's excellent X-ray attenuation coefficient. We then monitored and investigated their absorption, distribution, metabolism, and excretion. We also used CT imaging to demonstrate that Cu3BiS3 NDs can be quickly removed through renal clearance, which may be related to their small size, rapid chemical transformation, and degradation in an acidic lysosomal environment as characterized by synchrotron radiation-based X-ray absorption near-edge structure spectroscopy. These results reveal that Cu3BiS3 NDs act as a simple but powerful "theranostic" nanoplatform for MSOT/CT imaging-guided tumor ablation with excellent metabolism and rapid clearance that will improve safety for clinical applications in the future.

Independent control of electrical and heat conduction by nanostructure designing for Si-based thermoelectric materials.

The high electrical and drastically-low thermal conductivities, a vital goal for high performance thermoelectric (TE) materials, are achieved in Si-based nanoarchitecture composed of Si channel layers and epitaxial Ge nanodots (NDs) with ultrahigh areal density (~1012 cm−2). In this nanoarchitecture, the ultrasmall NDs and Si channel layers play roles of phonon scattering sources and electrical conduction channels, respectively. Electron conductivity in n-type nanoacrhitecture shows high values comparable to those of epitaxial Si films despite the existence of epitaxial NDs. This is because Ge NDs mainly scattered not electrons but phonons selectively, which could be attributed to the small conduction band offset at the epitaxially-grown Si/Ge interface and high transmission probability through stacking faults. These results demonstrate an independent control of thermal and electrical conduction for phonon-glass electron-crystal TE materials by nanostructure designing and the energetic and structural interface control.

Interface and strain effects on the H-sorption thermodynamics of size-selected Mg nanodots

This work deals with the thermodynamics of hydride formation in 3-D nanoconfined Mg. Two ensembles of nearly monodisperse Mg nanodots (NDs) with different diameters (60 and 320 nm), were grown by the template nanopatterning method, using ultra-thin alumina membranes (UTAMs) with ordered porosity as evaporation masks. Multilayer NDs consisting of 30 nm Mg, 5 nm Ti and 5 nm Pd were deposited on UTAM-coated glass substrates by molecular beam epitaxy. The lateral surface of the NDs is constituted by native MgO. The morphology of the NDs was characterized by field emission scanning electron microscopy and atomic force microscopy. Hydride formation and decomposition was studied at low temperature (363–393 K) by means of optical hydrogenography. Compared to bulk Mg, the plateau pressure for hydrogen absorption in NDs exhibits an upward shift, which is larger for small NDs. Differently, the desorption plateau pressure is almost the same for the two NDs size and is lower than for bulk Mg. These hydrogen sorption features are discussed in the frame of a model that takes into account both interface energy and elastic strain energy in the constrained nanodots. The onset of plastic deformation, marked by a high pressure hysteresis between hydrogen absorption and desorption isotherms, limits the extent of hydride destabilization that can be achieved by elastic strain engineering.

Growth of ErAs nanodots by molecular beam epitaxy for application to tunneling junctions in multijunction solar cells

ErAs nanodots (NDs) grown on GaAs(001) substrates by using molecular beam epitaxy (MBE) were investigated. Atomic force microscope images indicate that the size of ErAs NDs increases with deposition time and growth temperature. A calibration was performed to determine the deposition rate of ErAs in order that the size of NDs can be accurately controlled and hence optimized. Local current flow images and surface profiles around ErAs NDs were simultaneously measured to clarify the local conductivity distribution corresponding to a real space profile. Furthermore, we also fabricated and characterized an ErAs-ND-embedded GaAs tunnel junction (TJ), which resulted in a voltage drop of 30 mV for 15 A/cm2 operation current equivalent to 1000 suns concentration, which is less than one-third of that of a conventional heavily doped tunnel junction.

Quantitative Characterization of Mechanical Property of Annealed Monolayer Colloidal Crystal

Quantitative characterization of the mechanical properties of a polystyrene (PS) monolayer colloidal crystal (MCC) annealed with solvent vapor has been performed for the first time by means of atomic force microscopy nanoindentation. The results showed that both the compressive and bending elastic modulus of PS MCC increased with the prolongation of annealing time from initial to 13 min. When the annealing time reached 15 min or even more, the PS MCC almost deformed to a planar film, and the elastic modulus of the PS MCC presented a drastic increase. These results provide a basis for tailoring the mechanical properties of a polymer colloidal monolayer via solvent vapor annealing. Such self-supported and high-mechanical-strength colloidal monolayers can be transferred to other surfaces for potential and promising applications in the bottom-up fabrication of highly ordered nanostructured materials such as nano dot arrays, photonic crystals, and many others.

Aqueous phase preparation of ultrasmall MoSe2 nanodots for efficient photothermal therapy of cancer cells.

Photothermal therapy (PTT) is a promising cancer treatment with both high effectiveness and fewer side effects. However, an ideal PTT agent not only needs strong absorption of near-infrared (NIR) light and high photothermal conversion efficiency, but also needs good biocompatibility, stability, and small size, which makes the design and preparation of a novel PTT agent a great challenge. In this work, we developed an ultrasonication-assisted liquid exfoliation method for the direct preparation of ultrasmall (2-3 nm) MoSe2 nanodots (NDs) in aqueous solution and demonstrated their superior properties as a PTT agent. The as-prepared MoSe2 NDs have strong absorption of NIR light and high photothermal conversion efficiency of about 46.5%. In vitro cellular experiments demonstrate that MoSe2 NDs have negligible cytotoxicity and can efficiently kill HeLa cells (human cervical cell line) under NIR laser (785 nm) irradiation.

Correlation Effect of Two Donors in a Nano Dot under the Influence of Magneticfield

Correlation Effect of Two Donors in a Nano Dot under the Influence of Magneticfield

Fluorescent carbon nano dots from lignite: unveiling the impeccable evidence for quantum confinement.

Synthesizing nano carbon from its bulk precursors is of recent research interest. In this report, luminescent carbon nanoparticles (CNPs) with tunable particle size and surface functionality are fabricated from lignite using ethylenediamine as the reactive solvent and surface passivating agent via different experimental methods. From the steady-state and time-resolved photophysical studies of these differently sized CNPs, it is unveiled that the energy of the excitons generated after photoexcitation is quantum confined, and it influences the observed photophysical behaviour significantly only when the particle size is less than 10 nm. A larger size of the CNPs and less surface functionalization lead to aggregation, and quenching of the fluorescence. But by dispersing smaller size CNPs in sodium sulfate matrix exhibits fluorescence in the solid state with an absolute fluorescence quantum yield of ∼34%. The prospective application of this hybrid material in sensing and removal of moisture in the atmosphere is illustrated.

Organic-Inorganic Hybrid Ternary Bulk Heterojunction of Nanostructured Perovskite-Low Bandgap Polymer-PCBM for Improved Efficiency of Organic Solar Cells.

A new organic-inorganic ternary bulk heterojunction (TBHJ) hybrid configuration comprised of nanostructured (CH3)3NHPbI3 (MAPbI3) perovskite-low bandgap PCPDTBT-PCBM was investigated. Well-organized TBHJ films were readily prepared by sequential spin-casting of sparsely covered MAPbI3 nano dots and PCPDTBT-PCBM bulk heterojunction (BHJ) composites on ITO/PEDOT:PSS substrates. The TBHJ hybrid device configuration comprising diiooctane (DIO) treated MAPbI3 perovskite nano dots and a PCPDTBT-PCBM BHJ composite processed with DIO additive exhibited excellent performances. The DIO additive played a key role in developing perovskite structures of MAPbI3 nano dots and induced the (110) directional crystallinity growth of longitudinal constructive morphologies such as nano rods. The improved photocurrent and fill factor compared to those of conventional BHJ devices led to an increase in efficiency of ∼28%. This improved photovoltaic performance originated from the higher quantum efficiencies contributed by the charge transfer from nanostructured MAPbI3 perovskite to PCBM. These TBHJs composed of nanostructured MAPbI3 perovskite, PCPDTBT, and PCBM also facilitated the exciton dissociation in the multi-BHJ system between MAPbI3 perovskite, PCPDTBT, and PCBM.

Formation and characterization of high-density FeSi nanodots on SiO2 induced by remote H2 plasma

We demonstrated the formation of high-density iron silicide nanodots (NDs) on thermally grown SiO2 by exposing an electron-beam-evaporated Fe/amorphous-Si/Fe (Fe/a-Si/Fe) trilayer stack to remote H2 plasma without any external heating and characterized their silicidation state and crystalline phase. After the remote H2 plasma exposure, the formation of NDs with an areal density of ∼4.3 × 1011 cm−2 and an average height of ∼7.1 nm was confirmed. X-ray photoelectron spectroscopy (XPS) analyses indicate silicidation reaction induced by the remote H2 plasma exposure, which was accompanied by the agglomeration of Fe and Si atoms on the SiO2 surface. The formation of a crystalline β-FeSi2 phase was confirmed by Raman scattering spectroscopy and XRD pattern measurements. The electrical separation among the β-FeSi2 NDs was confirmed from changes in surface potential due to charging of the dots. The surface potential of the NDs changed in a stepwise manner with respect to the tip voltage because of multistep electron injection into and extraction from the semiconductor β-FeSi2 NDs.

Phonon transport control by nanoarchitecture including epitaxial Ge nanodots for Si-based thermoelectric materials.

Phonon transport in Si films was controlled using epitaxially-grown ultrasmall Ge nanodots (NDs) with ultrahigh density for the purpose of developing Si-based thermoelectric materials. The Si/Ge ND stacked structures, which were formed by the ultrathin SiO2 film technique, exhibited lower thermal conductivities than those of the conventional nanostructured SiGe bulk alloys, despite the stacked structures having a smaller Ge fraction. This came from the large thermal resistance caused by phonon scattering at the Si/Ge ND interfaces. The phonon scattering can be controlled by the Ge ND structure, which was independent of Si layer structure for carrier transport. These results demonstrate the effectiveness of ultrasmall epitaxial Ge NDs as phonon scattering sources, opening up a route for the realisation of Si-based thermoelectric materials.

Resistive Switching Characteristics of Si-Rich Oxides with Embedding Ti Nanodots

We have studied a formation of Ti nanodots (NDs) by remote-H2 plasma (H2-RP) treatment and an impact of Ti-NDs embedding into Si-rich oxide (SiOx) on the resistive switching behaviors because it is expected that NDs can trigger the formation of the conductive filament path in SiOx. Atomic force microscope (AFM) analysis shows that a combination of a Ge(~20 nm)/Ti(~3 nm) stacked layer with subsequent H2-RP exposure is effective to form the high-density Ti-NDs, where Ge layer is used as a barrier layer against oxidation of the ultrathin Ti layer. From X-ray photoelectron spectroscopy (XPS) analysis, an etching of Ge layer by H2-RP exposure and surface oxidation of Ti-NDs by air exposure are observed. After fabrication of SiOx-ReRAMs with the NDs, resistive switching characteristics are measured from current-voltage (I-V) curves and compared to the results obtained from the diodes without the NDs.

Highly repeatable synthesis of nHA with high aspect ratio

Hydroxyapatite with different aspect ratios such as nano dots, sheets, rods and fibers were synthesized by the hydrothermal and precipitation methods respectively. It is not difficult to synthesize nanohydroxyapatite (nHA) with a low aspect ratio using published methods. However, it is still a challenge to synthesize nHA particles with a high aspect ratio, such as nHA fibers, using a well-controlled and repeatable method. In this study, we describe a method to control the aspect ratio of nHA by controlling the initial pH of the solution. A highly repeatable synthesis method for nHA fibers has been developed, and the mechanism to control the aspect ratio is discussed.

Fabrication of single titanium oxide nanodot ultraviolet sensors by atomic force microscopy nanolithography

We report on the fabrication of single titanium oxide nanodot (ND) ultraviolet sensors by atomic force microscopy (AFM) nanolithography. A single titanium nanowire is first fabricated by AFM nanomachining and gold contact electrodes are then created by photolithography. By subsequent AFM nano-oxidation, a single titanium oxide ND sensor is produced. Two types of ND sensors, namely ohmic contact and Schottky contact, have been obtained and the sensitivities are around 0.25 and 320, respectively, under ultraviolet illumination. The rise and the reset times of the Schottky contact sensor are also significantly faster.

Formation of epitaxial nanodots on Si substrates with controlled interfaces and their application

We developed the epitaxial growth technique of various kinds of nanodots (NDs) on Si substrates using ultrathin SiO2 film. Fe-based NDs were epitaxially grown on Si substrates without intermixing with Si atoms in Si substrates despite the easy reaction of Si and Fe, resulting in atomically sharp interfaces between NDs and substrates. The status control of the nuclei was important at the nucleation stage in terms of control of ND crystal structure. The proper nuclei also suppressed the undesirable reactions such as intermixing of Fe and Si in Si substrates. The epitaxial NDs formed by our technique can supply the prospect for the application to some devices.