Quantum confinement and surface chemistry of 0.8–1.6 nm hydrosilylated silicon nanocrystals

In the framework of density functional theory （DFT）, we have studied the electronic properties of alkene/alkyne- hydrosilylated silicon nanocrystals （Si NCs） in the size range from 0.8 nm to 1.6 nm. Among the alkenes with all kinds of functional groups considered in this work, only those containing -NH2 and -C4H3S lead to significant hydrosilylation- induced changes in the gap between the highest occupied molecular orbital （HOMO） and the lowest unoccupied molecular orbital （LUMO） of an Si NC at the ground state. The quantum confinement effect is dominant for all of the alkene- hydrosilylated Si NCs at the ground state. At the excited state, the prevailing effect of surface chemistry only occurs at the smallest （0.8 nm） Si NCs hydrosilylated with alkenes containing -NH2 and -C4H3S. Although the alkyne hydrosilylation gives rise to a more significant surface chemistry effect than alkene hydrosilylation, the quantum confinement effect remains dominant for alkyne-hydrosilylated Si NCs at the ground state. However, at the excited state, the effect of surface chemistry induced by the hydrosilylation with conjugated alkynes is strong enough to prevail over that of quantum confinement.

Charge transport and quantum confinement in MoS2 dual-gated transistors

Semiconductive two dimensional(2D)materials have attracted significant research attention due to their rich band structures and promising potential for next-generation electrical devices.In this work,we investigate the MoS2 field-effect transistors(FETs)with a dual-gated(DG)architecture,which consists of symmetrical thickness for back gate(BG)and top gate(TG)dielectric.The thickness-dependent charge transport in our DG-MoS2 device is revealed by a four-terminal electrical measurement which excludes the contact influence,and the TCAD simulation is also applied to explain the experimental data.Our results indicate that the impact of quantum confinement effect plays an important role in the charge transport in the MoS2 channel,as it confines charge carriers in the center of the channel,which reduces the scattering and boosts the mobility compared to the single gating case.Furthermore,temperature-dependent transfer curves reveal that multi-layer MoS2 DG-FET is in the phonon-limited transport regime,while single layer MoS2 shows typical Coulomb impurity limited regime.

Quantum confinement effects and source-to-drain tunneling in ultra-scaled double-gate silicon n-MOSFETs

By using the linear combination of bulk band （LCBB） method incorporated with the top of the barrier splitting （TBS） model, we present a comprehensive study on the quantum confinement effects and the source-to-drain tunneling in the ultra-scaled double-gate （DG） metal-oxide semiconductor field-effect transistors （MOSFETs）. A critical body thickness value of 5 nm is found, below which severe valley splittings among different X valleys for the occupied charge density and the current contributions occur in ultra-thin silicon body structures. It is also found that the tunneling current could be nearly 100% with an ultra-scaled channel length. Different from the previous simulation results, it is found that the source-to-drain tunneling could be effectively suppressed in the ultra-thin body thickness （2.0 nm and below） by the quantum confinement and the tunneling could be suppressed down to below 5% when the channel length approaches 16 nm regardless of the body thickness.

Phonon thermal transport properties of XB_(2)(X=Mg and Al)compounds:considering quantum confinement and electron-phonon interaction

XB_(2)(X=Mg and Al)compounds have drawn great attention for their superior electronic characteristics and potential applications in semiconductors and superconductors.The study of phonon thermal transport properties of XB_(2)is significant to their application and mechanism behind research.In this work,the phonon thermal transport properties of three-dimensional(3D)and two-dimensional(2D)XB_(2)were studied by first-principles calculations.After considering the electron-phonon interaction(EPI),the thermal conductivities(TCs)of 3D Mg B_(2)and 3D Al B_(2)decrease by 29%and 16%which is consistent with experimental values.Moreover,the underlying mechanisms of reduction on lattice TCs are the decrease in phonon lifetime and heat capacity when considering quantum confinement effect.More importantly,we are surprised to find that there is a correlation between quantum confinement effect and EPI.The quantum confinement will change the phonon and electron characteristics which has an impact on EPI.Overall,our work is expected to provide insights into the phonon thermal transport properties of XB_(2)compounds considering EPI and quantum confinement effect.

Novel method to determine effective length of quantum confinement using fractional-dimension space approach

The binding energy and effective mass of a polaron confined in a GaAs film deposited on an AlGal-xAs substrate are investigated, for different film thickness values and aluminum concentra- tions and within the framework of the fractional-dimensional space approach. Using this scheme, we propose a new method to define the effective length of the quantum confinement. The limita- tions of the definition of the original effective well width are discussed, and the binding energy and effective mass of a polaron confined in a GaAs film are obtained. The fl-actional-dimensional theo- retical results are shown to be in good agreement with previous, more detailed calculations based on second-order perturbation theory.

Surface depletion field in 2D perovskite microplates: Structural phase transition, quantum confinement and Stark effect

Surface depletion field would introduce the depletion region near surface and thus could significantly alter the optical,electronic and optoelectronic properties of the materials,especially low-dimensional materials.Two-dimensional(2D)organic—inorganic hybrid perovskites with van der Waals bonds in the out-of-plane direction are expected to have less influence from the surface depletion field;nevertheless,studies on this remain elusive.Here we report on how the surface depletion field affects the structural phase transition,quantum confinement and Stark effect in 2D(BA)2PbI4 perovskite microplates by the thickness-,temperature-and power-dependent photoluminescence(PL)spectroscopy.Power dependent PL studies suggest that high-temperature phase(HTP)and low-temperature phase(LTP)can coexist in a wider temperature range depending on the thickness of the 2D perovskite microplates.With the decrease of the microplate thickness,the structural phase transition temperature first gradually decreases and then increases below 25 nm,in striking contrast to the conventional size dependent structural phase transition.Based on the thickness evolution of the emission peaks for both high-temperature phase and low-temperature phase,the anomalous size dependent phase transition could probably be ascribed to the surface depletion field and the surface energy difference between polymorphs.This explanation was further supported by the temperature dependent PL studies of the suspended microplates and encapsulated microplates with graphene and boron nitride flakes.Along with the thickness dependent phase transition,the emission energies of free excitons for both HTP and LTP with thickness can be ascribed to the surface depletion induced confinement and Stark effect.

Inorganic Halide Perovskite Quantum Dots:A Versatile Nanomaterial Platform for Electronic Applications

Metal halide perovskites have generated significant attention in recent years because of their extraordinary physical properties and photovoltaic performance.Among these,inorganic perovskite quantum dots(QDs)stand out for their prominent merits,such as quantum confinement effects,high photoluminescence quantum yield,and defect-tolerant structures.Additionally,ligand engineering and an all-inorganic composition lead to a robust platform for ambient-stable QD devices.This review presents the state-of-the-art research progress on inorganic perovskite QDs,emphasizing their electronic applications.In detail,the physical properties of inorganic perovskite QDs will be introduced first,followed by a discussion of synthesis methods and growth control.Afterwards,the emerging applications of inorganic perovskite QDs in electronics,including transistors and memories,will be presented.Finally,this review will provide an outlook on potential strategies for advancing inorganic perovskite QD technologies.

The impact of quantum confinement on the electrical characteristics of ultrathinchannel GeOI MOSFETs

The impact of quantum confinement on the electrical characteristics of ultrathin-channel GeO1 n- MOSFETs is investigated on the basis of the density-gradient model in TCAD software. The effects of the channel thickness （Tch） and back-gate bias （Vbg） on the electrical characteristics of GeOI MOSFETs are examined, and the simulated results are compared with those using the conventional semi-classical model. It is shown that when T^h 〉 8 rim, the electron conduction path of the GeOI MOSFET is closer to the front-gate interface under the QC model than under the CL model, and vice versa when Tch 〈 8 rim. Thus the electrically controlled ability of the front gate of the devices is influenced by the quantum effect. In addition, the quantum-mechanical mechanism will enhance the drain-induced barrier lowering effect, increase the threshold voltage and decrease the on-state current; for a short channel length （≤ 30 nm）, when Tch 〉 8 nm （or 〈 8 nm）, the quantum-mechanical mechanism mainly impacts the subthreshold slope （or the threshold voltage）. Due to the quantum-size effect, the off-state current can be suppressed as the channel thickness decreases.

From self-assembly to quantum guiding: A review of magnetic atomic structures on noble metal surfaces

Recent advances in the study of magnetic atomic structures on noble metal surfaces are reviewed. These include one- dimensional strings, two-dimensional hexagonal superlattices, and novel structures stabilized by quantum guiding. The combined techniques of low-temperature scanning tunneling microscopy, kinetic Monte Carlo simulations, and ab initio calculations reveal that surface-state-mediated adatom-step and adatom-adatom interactions are the driving forces for self- assembly of these structures. The formation conditions are further discussed by comparing various experimental systems and the kinetic Monte Carlo simulations. Using scanning tunneling spectroscopy and tight-binding calculations together, we reveal that the spectra of these well-ordered structures have characteristic peaks induced by electronic scattering processes of the atoms within the local environment. Moreover, it is demonstrated that quantum confinement by means of nano-size corrals has significant influence on adatom diffusion and self-assembly, leading to a quantum-guided self-assembly.

Tight-binding study of quantum transport in nanoscale GaAs Schottky MOSFET

This paper explores the band structure effect to elucidate the feasibility of an ultra-scaled GaAs Schottky MOSFET （SBFET） in a nanoscale regime. We have employed a 20-band sp3dSs＊ tight-binding （TB） approach to compute E - K dis- persion. The considerable difference between the extracted effective masses from the TB approach and bulk values implies that quantum confinement affects the device performance. Beside high injection velocity, the ultra-scaled GaAs SBFET suffers from a low conduction band DOS in the F valley that results in serious degradation of the gate capacitance. Quan- tum confinement also results in an increment of the effective Schottky barrier height （SBH）. Enhanced Schottky barriers form a double barrier potential well along the channel that leads to resonant tunneling and alters the normal operation of the SBFET. Major factors that may lead to resonant tunneling are investigated. Resonant tunneling occurs at low temperatures and low drain voltages, and gradually diminishes as the channel thickness and the gate length scale down. Accordingly, the GaAs （100） SBFET has poor ballistic performance in nanoscale regime.

On the binding energies of excitons in polar quantum well structures in a weak electric field

The binding energies of excitons in quantum well structures subjected to an applied uniform electric field by taking into account the exciton longitudinal optical phonon interaction is calculated. The binding energies and corresponding Stark shifts for Ⅲ-Ⅴ and Ⅱ-Ⅵ compound semiconductor quantum well structures have been numerically computed. The results for GaAs/A1GaAs and ZnCdSe/ZnSe quantum wells are given and discussed. Theoretical results show that the exciton-phonon coupling reduces both the exciton binding energies and the Stark shifts by screening the Coulomb interaction. This effect is observable experimentally and cannot be neglected.

Recent Progress of Layered Perovskite Solar Cells Incorporating Aromatic Spacers

Layered two dimensional(2D) or quasi-2D perovskites are emerging photovoltaic materials due to their superior environment and structure stability in comparison with their 3D counterparts. The typical 2D perovskites can be obtained by cutting 3D perovskites along < 100 > orientation by incorporation of bulky organic spacers, which play a key role in the performance of 2D perovskite solar cells(PSCs). Compared with aliphatic spacers, aromatic spacers with high dielectric constant have the potential to decrease the dielectric and quantum confinement effect of 2D perovskites, promote efficient charge transport and reduce the exciton binding energy, all of which are beneficial for the photovoltaic performance of 2D PSCs. In this review, we aim to provide useful guidelines for the design of aromatic spacers for 2D perovskites. We systematically reviewed the recent progress of aromatic spacers used in 2D PSCs. Finally, we propose the possible design strategies for aromatic spacers that may lead to more efficient and stable 2D PSCs.

Silicon nanoparticles: Preparation, properties, and applications

Silicon nanoparticles have attracted great attention in the past decades because of their intriguing physical properties, active surface state, distinctive photoluminescence and biocompatibility. In this review, we present some of the recent progress in preparation methodologies and surface functionalization approaches of silicon nanoparticles. Further, their promising applications in the fields of energy and electronic engineering are introduced.

Multipeak-structured photoluminescence mechanisms of as-prepared and oxidized Si nanoporous pillar arrays

Silicon dominates the electronic industry, but its poor optical properties mean that it is not preferred for photonic applications. Visible photoluminescence （PL） was observed from porous Si at room temperature in 1990, but the origin of these light emissions is still not fully understood. This paper reports that an Si nanocrystal, silicon nanoporous pillar array （Si-NPA） with strong visible PL has been prepared on a Si wafer substrate by the hydrothermal etching method. After annealing in 02 atmosphere, the hydride coverage of the Si pillar internal surface is replaced by an oxide layer, which comprises of a great quantity of Si nanocrystal （nc-Si） particles and each of them axe encapsulated by an Si oxide layer. Meanwhile a transition from efficient triple-peak PL bands from blue to red before annealing to strong double-peak blue PL bands after annealing is observed. Comparison of the structural, absorption and luminescence characteristics of the as-prepared and oxidized samples provides evidence for two competitive transition processes, the band-to-band recombination of the quantum confinement effect of nc-Si and the radiative recombination of excitons from the luminescent centres located at the surface of nc-Si units or in the Si oxide layers that cover the nc-Si units because of the different oxidation degrees. The sizes of nc-Si and the quality of the Si oxide surface are two major factors affecting two competitive processes. The smaller the size of nc-Si is and the stronger the oxidation degree of Si oxide layer is, the more beneficial for the luminescent centre recombination process to surpass the quantum confinement process is. The clarification on the origin of the photons may be important for the Si nanoporous pillar array to control both the PL band positions and the relative intensities according to future device requirements and further fabrication of optoelectronic nanodevices.

Photoluminescence Properties of Nanocrystalline 3C-SiC Films

Nanocrystalline （nc） 3C-SiC films on the Si substrate were prepared by the helicon wave plasma enhanced chemical vapor deposition （HW-PECVD） technique. With the SiH4-CH4 gas flow ratio changing, the films exhibit different photoluminescence （PL） characteristics. Under the stoichiometric condition, the PL peak redshift from 470 nm to 515 nm is detected with the increase of excitation wavelength, which can be attributed to the quantum confinement effect radiation of 3C-SiC nanocrystals of different sizes. However, the appearance of an additional PL band at 436 nm in Si-rich film might be sourced back to the excess of Si defect centers in it. This is also the case for C-rich film for its PL band lying at 570 nm. The results above quoted indicate an important influence of gas flow ratio on the PL properties of the SiC films providing an effective guidance for analyzing the luminescence mechanism and exploring the high-efficiency light emission of the SiC films.

Influence of surface scattering on the thermal properties of spatially confined GaN nanofilm

Gallium nitride（GaN）, the notable representative of third generation semiconductors, has been widely applied to optoelectronic and microelectronic devices due to its excellent physical and chemical properties. In this paper, we investigate the surface scattering effect on the thermal properties of GaN nanofilms. The contribution of surface scattering to phonon transport is involved in solving a Boltzmann transport equation（BTE）. The confined phonon properties of GaN nanofilms are calculated based on the elastic model. The theoretical results show that the surface scattering effect can modify the cross-plane phonon thermal conductivity of GaN nanostructures completely, resulting in the significant change of size effect on the conductivity in GaN nanofilm. Compared with the quantum confinement effect, the surface scattering leads to the order-of-magnitude reduction of the cross-plane thermal conductivity in GaN nanofilm. This work could be helpful for controlling the thermal properties of Ga N nanostructures in nanoelectronic devices through surface engineering.

Band structure of silicon and germanium thin films based on first principles

In nanomaterials, optical anisotropies reveal a fundamental relationship between structural and optical properties, in which directional optical properties can be exploited to enhance the performance of optoelectronic devices. First principles calculation based on density functional theory （DFT） with the generalized gradient approximation （GGA） are carried out to investigate the energy band gap structure on silicon （Si） and germanium （Ge） nanofilms. Simulation results show that the band gaps in Si （100） and Ge （111） nanofilms become the direct-gap structure in the thickness range less than 7.64 nm and 7.25 nm respectively, but the band gaps of Si （111） and Ge （110） nanofilms still keep in an indirect-gap structure and are independent on film thickness, and the band gaps of Si （110） and Ge （100） nanofilms could be transferred into the direct-gap structure in nanofilms with smaller thickness. It is amazing that the band gaps of Si（1-x）/ZGexSi（1-x）/2 sandwich structure become the direct-gap structure in a certain area whether （111） or （100） surface. The band structure change of Si and Ge thin films in three orientations is not the same and the physical mechanism is very interesting, where the changes of the band gaps on the Si and Ge nanofilms follow the quantum confinement effects.

Metallic Graphene Nanoribbons

Isolated graphene nanoribbons(GNRs)usually have energy gaps,which scale with their widths,owing to the lateral quantum confinement effect of GNRs.The absence of metallic GNRs limits their applications in device interconnects or being one-dimensional physics platform to research amazing properties based on metallicity.A recent study published in Science provided a novel method to produce metallic GNRs by inserting a symmetric superlattice into other semiconductive GNRs.This finding will broader the applications of GNRs both in nanoelectronics and fundamental science.

Transient spectral and dynamic properties of magic-size Cd_(3)P_(2) nanoclusters in the limit of strong confinement

Both semiconductor nanocrystals and organic molecules are important photofunctional materials for an array of applications.It is interesting to examine the intermediate regime between these two families,which can be interpreted as the strong-confinement limit of the nanocrystals or alternatively as the large-size limit of molecules.Here,we choose Cd_(3)P_(2) magic-size clusters(MSCs)as a unique platform and apply time-resolved spectroscopy to investigate their spectral and dynamic properties.We find that these small clusters display molecular-like vibronic progression on their absorption and emission spectra and a large Stokes shift,which leads to well-separated transient absorption bleach and stimulated emission signals distinct from typical nanocrystals.On the other hand,such small size MSCs can still accommodate biexciton states,and the strongly enhanced Coulombic interactions lead to very fast dephasing of the biexciton resonance as well as rapid biexciton Auger annihilation(1.5 ps).Further,temperature-dependent measurements provide evidence for the transformation of band-edge excitons to localized excitons,with the localization likely driven by the softened lattice in these small-size clusters.These collective results demonstrate that strongly-confined nanoclusters indeed bridge the gap between nanocrystals and molecules,and can be a unique library to search for exotic excited state properties.

Physical Parameter Variation Analysis on the Performance Characteristics of Nano DG-MOSFETs

DG-MOSFETs are the most widely explored device architectures for nano-scale CMOS circuit design in sub-50 nm due to the improved subthreshold slope and the reduced leakage power compared to bulk MOSFETs. In thin-film (tsi < 10 nm) DG-MOS structures, charge carriers are affected by tsi- induced quantum confinement along with the confinement caused by a very high electric field at the interface. Therefore, quantum confinement effects on the device characteristics are also quite important and it needs to be incorporated along with short channel effects for nano-scale circuit design. In this paper, we analyzed a DG-MOSFET structure at the 20 nm technology node incorporating quantum confinement effects and various short channel effects. The effect of physical parameter variations on performance characteristics of the device such as threshold voltage, subthreshold slope, ION - IOFF ratio, DIBL, etc. has been investigated and plotted through extensive TCAD simulations. The physical parameters considered in this paper are operating temperature (Top), channel doping concentration (Nc), gate oxide thickness (tox) and Silicon film thickness (tsi). It was observed that quantum confinement of charge carriers significantly affected the performance characteristics (mostly the subthreshold characteristics) of the device and therefore, it cannot be ignored in the subthreshold region-based circuit design like in many previous research works. The ATLASTM device simulator has been used in this paper to perform simulation and parameter extraction. The TCAD analysis presented in the manuscript can be incorporated for device modeling and device matching. It can be used to illustrate exact device behavior and for proper device control.