Multilayer Nanocrystals Research Articles

Concentration-Dependent Layer-Stacking and the Influence on Phase-Conversion in Colloidally Synthesized WSe2 Nanocrystals.

We report a synthesis of WSe2 nanocrystals in which the number of layers is controlled by varying the precursor concentration. By altering the ratios and concentrations of W(CO)6 and Ph2Se2 in trioctylphosphine oxide, we show that high [Se] and large Se/W ratios lead to an increased number of layers per nanocrystal. As the number of layers per nanocrystal is increased, the nanocrystal ensembles show less phase-conversion from the metastable 2M phase to the thermodynamically favored 2H phase. Density functional theory calculations indicate that the interlayer binding energy increases with the number of layers, indicating that the stronger interlayer interactions in multilayered nanocrystals may increase the energy barrier to phase-conversion. The results presented herein provide insights for directing phase-conversion in solution-phase syntheses of transition metal dichalcogenides.

Three primary color emissions from single multilayered nanocrystals.

The achievement of three-primary-color luminescence in a single material will lead to revolutionary developments of many advanced applications such as dynamic display with ultra-high resolution, and complex anti-counterfeiting. Here we report the realization of steady-state three-primary-color emission in single multilayered NaYF4 upconversion (UC) nanoparticles. In this core-shell structure, a novel design of a tri-sensitizer, i.e., Nd3+, Yb3+ and Er3+ ions, is utilized, which effectively absorbs the excitation photons of 808, 980 and 1550 nm, and then exhibits blue, red and green emissions, respectively. By simply combining the three primary color emissions, tunable full-color luminescence was achieved in this single material. These nanoparticles have demonstrated promising potential applications in dynamic display and multiple anti-counterfeiting.

Synthesis, characterization and up-conversion luminescence properties of alpha-NaYF4:Yb3+/Er3+/PVP/MOFs multilayer nanocrystals

The precursor PVP-capped α-NaYF4:Yb3+/Er3+ nanospheres were used as the templates for preparing the α-NaYF4:Yb3+/Er3+/PVP/MOFs multilayer nanocrystals with a self-template method. By using iron (III) carboxylate and zeolitic imidazolate frameworks dissolved in dimethylformamide (DMF) solution containing 25% of diethylene glycol (DEG), the spherical-shaped α-NaYF4:Yb3+/Er3+/PVP/MIL-100 and α-NaYF4:Yb3+/Er3+/PVP/ZIF-8 multilayer nanocrystals were successfully prepared with the sizes of 300-500 nm at 100oC for one hour. Under a 976 nm laser excitation at room temperature, the α-NaYF4:Yb3+/Er3+/PVP/MIL-100 and α-NaYF4:Yb3+/Er3+/PVP/ZIF-8 multilayer nanocrystals exhibited strong up-conversion luminescence with three emission bands centered at around 520 nm, 540 nm, and 655 nm corresponding to 2H11/2 → 4I15/2, 4S3/2 → 4I15/2, and 4F9/2 → 4I15/2transitions of Er3+ ions, respectively.

Multi-layered metal nanocrystals in a sol-gel spin-on-glass matrix for flash memory applications

A simple and low-cost process of embedding metal nanocrystals as charge storage centers within a dielectric is demonstrated to address leakage issues associated with the scaling of the tunnelling oxide in flash memories. Metal nanocrystals with high work functions (nickel, platinum and palladium) were prepared as embedded species in methyl siloxane spin-on-glass (SOG) films on silicon substrates. Sub-10 nm-sized, well-isolated, uniformly distributed, multi-layered nanocrystals with high particle densities (1011–1012 cm−2) were formed in the films by thermal curing of the spin-coated SOG films containing the metal precursors. Capacitance-Voltage measurements performed on metal-insulator-semiconductor capacitors with the SOG films show that the presence of metal nanocrystals enhanced the memory window of the films to 2.32 V at low operating voltages of ±5 V. These SOG films demonstrated the ability to store both holes and electrons. Capacitance-time measurements show good charge retention of more than 75% after 104 s of discharging. This work demonstrates the applicability of the low-cost in-situ sol-gel preparation in contrast to conventional methods that involve multiple and expensive processing steps.

Multilayer Ge nanocrystals embedded within Al2O3 matrix for high performance floating gate memory devices

Metal-insulator-silicon devices with Ge nanocrystals dispersed in Al2O3 have been studied with a view to exploit them for floating gate memory applications. Multilayer devices comprising of five layers Ge nanocrystals have exhibited superior memory characteristics over the single layer Ge and multilayer Si nanocrystals reported in literature. The effect of interface traps on the memory behavior using frequency dependent capacitance- and conductance-voltage measurements has been investigated. This study has demonstrated an enhanced memory window with superior retention characteristics, owing to the Coulomb blockade effect, due to the introduction of multi-layer nanocrystals in the floating gate.

Growth Mechanism for Single- and Multi-Layer MoS2Nanocrystals

Transmission electron microscopy (TEM) is used to study growth of MoS2 nanocrystals in situ. The nanocrystals are formed from a submonolayer molybdenum oxide dispersed on an oxide support by sulfidation in an H2S/H2 atmosphere. From series of time-resolved TEM images, it is revealed that single-layer MoS2 nanocrystals form preferentially and that multi-layer nanocrystals form late in the sulfidation process. The TEM images pinpoint that step sites in the support can act as nucleation centers for single-layer nanocrystals and that single-layer nanocrystals grow along the support surface. Moreover, the TEM images reveal that multi-layer MoS2 nanocrystals form in a layer-by-layer mode by the homogeneous nucleation of additional MoS2 layers onto already formed single-layer MoS2 nanocrystals. Hereby, the atomic-scale observations suggest that the formation of multi-layer MoS2 nanocrystals is an energetically more activated process than growth of single-layers. These findings explain why process parameters, such as temperature, can tune the relative fraction of single- to multi-layer MoS2 nanocrystals, which is important for their use in, e.g., hydrotreating catalysis.

Layer by layer growth of CdTe nanocrystal thin films and its photoluminescence

We synthesized size-controlled，uniformly distributed, and thioglycolic-stabilized CdTe nanocrystals by wet chemistry method. The multilayer nanocrystals were assembled on Si substrate by bifunctional linker molecule and the resonance energy transfer between different sized CdTe nanocrystals embedded in gelatin film was investigated. The results showed that the multilayer nanocrystals were in a good order because the distance between the layers was nearly the same. The distance between different sized nanocrystals was relatively short in the directly dried sample. We can observe the resonance energy transfer by the red shift in its photoluminescence.

Preparation of ZnS/CdSe/ZnS Quantum Dot Quantum Well by Using a Microfluidic Reactor

We have synthesized ZnS/CdSe/ZnS quantum dot quantum well (QDQW) by using a microfluidic reactor. The synthesized QDQW emitted blue fluorescence and the quantum yield was achieved at 50%. Furthermore, we were able to tune the photoluminescence wavelength while maintaining the full width at half maximum (FWHM) by controlling the flow rate in CdSe layer deposition process. Our results demonstrate that high quality CdSe phase was embedded in the ZnS core and shell, and the layer thickness was controlled homogeneously by using the microfluidic system. So, this system is considered as a useful tool for controlling the multilayer nanocrystals such as QDQW.

Formation and nonvolatile memory characteristics of multilayer nickel-silicide NCs embedded in nitride layer

The authors provided the formation and memory effects of nonvolatile multilayer nickel-silicide nanocrystal memory in this study. This proposed structure can efficiently improve the drawbacks of current floating gate and single-layer nanocrystal memories for the next-generation nonvolatile memory application. The charge trapping layer of multilayer structure was deposited by sputtering a commixed target (Ni0.3Si0.7) in the argon and nitrogen ambiance, and then used a low temperature rapid thermal annealing to form uniform nanocrystals. Transmission electron microscope images clearly show the multilayer and single-layer nanocrystal structures embedded in SiNx. X-ray photoelectron spectroscopy and x-ray diffraction also present the chemical states and crystallization of nanocrystals under different annealing temperature treatments. The capacitor with different memory structures was also studied and exhibited hysteresis characteristics after electrical operation. In addition, the multilayer nanocrystals revealed better charge storage ability and reliability than the single-layer nanocrystals.

Memory Characteristics of Atomic-Layer-Deposited High-κ HfAlO Nanocrystal Capacitors

The memory characteristics of atomic-layer-deposited high-K HfAlO nanocrystals in a p-Si/SiO 2 /CHfO 2 /Al 2 O 3 ]/Al 2 O 3 /platinum structure have been investigated. After the annealing treatment, the high-K HfAlO nanocrystals with a small diameter of 5 X 10 11 cm 2 have been observed by high-resolution transmission electron microscopy. A large hysteresis memory window of ∼ 10.4 V has been obtained. The high-K HfAlO nanocrystal memory capacitor with a small capacitance equivalent thickness of ∼ 8.5 ± 0.5 nm shows a small leakage current density of ∼ 22 μA cm 2 at a gate voltage of -16 V. A large memory window of ∼8 V has also been observed after 10 5 s of retention, due to the charge confinement in the high-K HfAlO multilayer nanocrystals.

Using double layer CoSi2 nanocrystals to improve the memory effects of nonvolatile memory devices

The nonvolatile memory device with multilayer nanocrystals has advantages such as the memory effects can be increased by the increasing density of the nanocrystals and the whole retention characteristic can be improved. There are much more electrons that can be stored in the double layer than single layer nanocrystal memory device. The double layer CoSi2 nanocrystals have better retention characteristic than the single layer. The good retention characteristic of the double layer device is due to the Coulomb-blockage effects on the top layer nanocrystals from the bottom layer nanocrystals. So, the memory effects of the nonvolatile memory device can be improved by using the double layer nanocrystals.

Stark effect in semiconductor nanocrystals: tight-binding approach

The influence of an external uniform electric field on the electron energy structure and charge densities of semiconductor nanocrystals is investigated. We study both uniform and multilayer nanocrystals as well as artificial molecules, i.e. systems built of two coupled quantum dots. We work in the framework of an empirical tight-binding theory, that allows the study of effects not accessible within the effective mass approach. The electric field can destroy the delocalized character of bonding-like and antibonding-like states of artificial molecules but can also lead to the formation of other field-tunable states.

Chains of Artificial Atoms in a Magnetic Field

The energy structure of one-dimensional chains of spherical nanocrystals is investigated. Both, uniform and multilayer nanocrystals are studied. Calculations are performed with the k·p method and the envelope function approximation (EFA). The valence subband mixing is accounted for by considering a four-band Hamiltonian. The high (spherical) symmetry of the building blocks leads to a unique energy structure with miniband widths depending on the z-component of the angular momentum. These minibands are (2L + 1) quasi-degenerated in the low-coupling regime, L being the quantum number of the angular momentum. The magnetic field splits quasidegeneracy leading to a mixture of successive narrow and wide minibands. The ground miniband of chains built of antidots becomes very narrow in the strong coupling regime and separates from the rest of the spectrum, so that a strong luminescence red-shift is predicted.

Multilayered Nanoheterostructures: Theory and Experiment

A series of multilayered nanocrystals consisting of alternating CdS and HgS layers has been synthesized. Those structures have been characterized by UV/VIS/NIR absorption and emission spectroscopy as well as transmission electron microscopy. Results of the optical spectroscopy are compared to theoretical calculation (“effective mass approximation”) and TEM results are compared to TEM simulations. Furthermore, comparable structures consisting of ZnS and CdS have been synthesized and characterized by optical spectroscopy and powder X-ray diffractometry.

Nanocrystal molecules and chains

The electron energy structure of linear artificial molecules and one-dimensional chains formed of spherical semiconductor nanocrystals is investigated with and without an applied magnetic field. Both uniform and multilayer nanocrystals are studied. The calculations are performed within the effective mass model by numerically integrating the effective mass equation on a two-dimensional cylindrical grid. Some calculations are, for comparison, performed also in the tight-binding approach. Density contours are presented to illustrate the transformation of states in systems of strongly interacting coupled quantum dots. Strong interaction between the quantum-dot–quantum-well structures in a chain of nanocrystals can lead to the formation of a very narrow ground-state miniband, well separated from the excited levels with the energies almost independent of the magnetic field.