Quantum Confinement Of Electrons Research Articles

Effects of Electron Quantum Confinement on Velocity Overshoot in Si Nanosheet Gate-All-Around FETs

Effects of Electron Quantum Confinement on Velocity Overshoot in Si Nanosheet Gate-All-Around FETs

Polyvinyl alcohol stabilized ZnS nanoaggregates using organosulfur source for photocatalytic disintegration of methyl orange dye

ZnS nanoaggregates surface-passivated by a hydroxyl-based polymer matrix, polyvinyl alcohol (PVA), have been synthesized using a wet chemical method and an organosulphur precursor. The nanocomposite was characterized by XRD, SEM, and FTIR. XRD patterns revealed wurtzite structure. FTIR spectrum confirmed the interaction of hydroxyl group of PVA with the ZnS. We proposed a probable pathway for the precipitation of zinc sulfide by alkaline hydrolysis of complex formed from (NH2)2CS and Zn2+-polymer complex. The optical properties were monitored using UV–Vis spectroscopy and photoluminescence (PL) spectroscopy. Quantum confinement of electrons is confirmed by increase in the optical band gap and determined to be 3.91 eV. The photocatalytic activity of the ZnS/PVA nanoaggregates particles was examined using methyl orange (MO). After 3 h of illumination, the photocatalytic degradation rate of methyl orange (MO) is about 94.20 %.

Electric field effect on the intersubband optical absorption of GeSn quantum wells with parabolically graded barriers

In this study, we propose a theoretical simulation of a Ge0.9Sn0.1 rectangular SQW with GeSn parabolically graded barriers (PGBs) in the terahertz (THz) region. The discrete intra-band confined energy levels and their matching wave functions were calculated by solving the stationary Schrödinger equation by the finite difference method taking into account the Compact Density Matrix approach under the framework of both the effective mass and the envelope wave function approximations. In this work, we studied the effect of the quantum square-well width on the intersubband transition and oscillator strength in a GeSn-strained-based Barrier-Well-Barrier GeSn/(Ge,α-Sn) PGBs to obtain the optimum quantum confinement of electrons. The electronic states and their wave functions in the conduction band were computed by solving the Schrödinger equation without and under the effect of an applied external electric field at room temperature. We then investigated the effect of the electric field on the optical absorption coefficient (OAC). Our numerical results show that for external fields (>15 kV/cm), an intersubband transitions (ISBTs) frequency band of 2–14 THz (8–58 meV) was obtained for the specific optimized parameters. These results should be beneficial to the design of devices based on GeSn QWs with PGB structures operating in the THz frequency range.

Metal sulfide sub-nanometer clusters formed within calix(8)arene Langmuir-Blodgett films

This work describes the construction of layered Langmuir-Blodgett (LB) films of a calixarene and the use of these as matrices for the synthesis of a range of metal sulfide nanoparticles. CuS, CdS, HgS, and PbS nano-clusters were formed within LB films of an octa-tertbutyl-calix(8)arene substituted with carboxylic acid groups deposited on different substrates (glass, quartz, and silicon) from either: (i) aqueous sub-phases containing 0.5 mM of the respective metal chloride salt (e.g. CuCl2, CdCl2, HgCl2, PbCl2), or (ii) by soaking the LB films in 10 mM solutions of the above salts for 1 h. The formation of metal-sulfide (MeS) nanoparticles was then achieved by exposing samples to H2S gas for 10–12 h.Deposition from salt containing subphases was more reliable and resulted in stoichiometric metal sulfides (CdS, HgS, PbS) being formed within LB films of the calix(8)arene carboxylic acid whereas Cu tended to form polysulfides. UV–vis absorption spectroscopy showed the presence of multiple absorption bands corresponding to electron transitions between energy levels in nanoclusters formed as result of quantum confinement of electrons and holes. The MeS clusters obtained by this process are amongst the smallest reported for LB films, being of the range 0.6–1.2 nm.

Quantum Confinement and Electronic Structure at the Surface of van der Waals Ferroelectric α-In2Se3.

Two-dimensional (2D) ferroelectric (FE) materials are promising compounds for next-generation nonvolatile memories due to their low energy consumption and high endurance. Among them, α-In2Se3 has drawn particular attention due to its in- and out-of-plane ferroelectricity, whose robustness has been demonstrated down to the monolayer limit. This is a relatively uncommon behavior since most bulk FE materials lose their ferroelectric character at the 2D limit due to the depolarization field. Using angle resolved photoemission spectroscopy (ARPES), we unveil another unusual 2D phenomenon appearing in 2H α-In2Se3 single crystals, the occurrence of a highly metallic two-dimensional electron gas (2DEG) at the surface of vacuum-cleaved crystals. This 2DEG exhibits two confined states, which correspond to an electron density of approximately 1013 electrons/cm2, also confirmed by thermoelectric measurements. Combination of ARPES and density functional theory (DFT) calculations reveals a direct band gap of energy equal to 1.3 ± 0.1 eV, with the bottom of the conduction band localized at the center of the Brillouin zone, just below the Fermi level. Such strong n-type doping further supports the quantum confinement of electrons and the formation of the 2DEG.

The inner barrier mediated spin–orbit control in stepped quantum wells

The inner barrier in stepped quantum wells, which embrace an intrinsic structural inversion asymmetry (SIA), offers more possibilities for the multi-band spin–orbit (SO) control. By varying the inner band offset [Formula: see text], which adjusts the quantum confinement for electrons and also the SIA, we reveal that both the Rashba ([Formula: see text]) and Dresselhaus ([Formula: see text]) SO coeffcients of the two subbands exhibit contrasting dependence on [Formula: see text]. Also, [Formula: see text] and [Formula: see text] may have opposite signs, depending on the value of [Formula: see text]. By fine tuning the inner offset, there may even exist a scenario in which [Formula: see text] identically vanishes but [Formula: see text] is finite, offering a knob on how to subband-selectively suppress spin relaxation induced by SO coupling. In addition, by resorting to an external gate [Formula: see text], we reveal that in the regime of [Formula: see text] and [Formula: see text] having opposite signs, their magnitudes also vary with [Formula: see text] in an opposite manner. This, together with the engineered inner barrier of stepped wells, will facilitate flexible spin manipulation. Finally, the interband SO couplings, which depend on the spatial overlap of wave functions of distinct subbands and the corresponding parities, are also discussed.

Graphene nanoribbons with hBN passivated edges grown by high-temperature molecular beam epitaxy

Integration of graphene and hexagonal boron nitride (hBN) in lateral heterostructures has provided a route to broadly engineer the material properties by quantum confinement of electrons or introduction of novel electronic and magnetic states at the interface. In this work we demonstrate lateral heteroepitaxial growth of graphene nanoribbons (GNRs) passivated by hBN using high-temperature molecular beam epitaxy (HT-MBE) to grow graphene in oriented hBN trenches formed ex-situ by catalytic nanoparticle etching. High-resolution atomic force microscopy (AFM) reveals that GNRs grow epitaxially from the etched hBN edges, and merge to form a GNR network passivated by hBN. Using conductive AFM we probe the nanoscale electrical properties of the nanoribbons and observe quasiparticle interference patterns caused by intervalley scattering at the graphene/hBN interface, which carries implications for the potential transport characteristics of hBN passivated GNR devices.

Carrier-Density Control of the Quantum-Confined 1T-TiSe_{2} Charge Density Wave.

Using angle-resolved photoemission spectroscopy, combined with first principle and coupled self-consistent Poisson-Schrödinger calculations, we demonstrate that potassium (K) atoms adsorbed on the low-temperature phase of 1T-TiSe_{2} induce the creation of a two-dimensional electron gas (2DEG) and quantum confinement of its charge-density wave (CDW) at the surface. By further changing the K coverage, we tune the carrier density within the 2DEG that allows us to nullify, at the surface, the electronic energy gain due to exciton condensation in the CDW phase while preserving a long-range structural order. Our Letter constitutes a prime example of a controlled exciton-related many-body quantum state in reduced dimensionality by alkali-metal dosing.

Recent Progress of Non-Cadmium and Organic Quantum Dots for Optoelectronic Applications with a Focus on Photodetector Devices

Quantum dots (QDs) are zero-dimensional (0D) nanomaterials with charge confinement in all directions that significantly impact various applications. Metal-free organic quantum dots have fascinating properties such as size-dependent bandgap tunability, good optical absorption coefficient, tunability of absorption and emission wavelength, and low-cost synthesis. Due to the extremely small scale of the materials, these characteristics originated from the quantum confinement of electrons. This review will briefly discuss the use of QDs in solar cells and quantum dots lasers, followed by a more in-depth discussion of QD application in photodetectors. Various types of metallic materials, such as lead sulfide and indium arsenide, as well as nonmetallic materials, such as graphene and carbon nanotubes, will be discussed, along with the detection mechanism.

Effect of Confinement and Coulomb Interactions on the Electronic Structure of the (111) LaAlO3/SrTiO3 Interface.

A tight binding supercell approach is used for the calculation of the electronic structure of the (111) LaAlO3/SrTiO3 interface. The confinement potential at the interface is evaluated solving a discrete Poisson equation by means of an iterative method. In addition to the effect of the confinement, local Hubbard electron-electron terms are included at the mean-field level within a fully self-consistent procedure. The calculation carefully describes how the two-dimensional electron gas arises from the quantum confinement of electrons near the interface due to the band bending potential. The resulting electronic sub-bands and Fermi surfaces show full agreement with the electronic structure determined by angle-resolved photoelectron spectroscopy experiments. In particular, we analyse how the effect of local Hubbard interactions change the density distribution over the layers from the interface to the bulk. Interestingly, the two-dimensional electron gas at the interface is not depleted by local Hubbard interactions which indeed induce an enhancement of the electron density between the first layers and the bulk.

Quantum confinement of electrons at metal surfaces

Scanning tunneling microscopy and spectroscopy experiments on surface-localized electron states confined to nanometer-scaled resonators are reviewed from the first observations to the recently discovered novel reflection mechanism of electron de Broglie waves. The focus of the presented work is on lateral confinement and on processes leading to finite decay rates of the confined states.

Researches and Applications of Nanostructured 2D Materials growth By Continuous Wavelength Laser Process

Two-dimensional (2D) nanomaterials, such as graphene and hexagonal boron nitride (hBN), have recently attracted increasing attention of researchers due to their unique phonon and electronic structures. These nanomaterials have been widely used in applications, such as sensors, catalysts, and optoelectronic devices, due to the lack of interlayer bonds and the quantum confinement of electrons. There have been several well studied methods to synthesis 2D materials including chemical vapor deposition(CVD), physical vapor deposition(PVD), liquid exfoliation, Li-intercalation exfoliation,wet chemistry,ultrasonic-ball milling, Among them, CVD is the most famous method to fabricate electronic device with high quality materials characteristic. However, CVD processes will cause extremely harsh heat budget to substrate during high temperature process. Recently, researchers have established transfer method fabricating device on other undamaged substrate. It is the trickiest problem make it cannot apply to industry.In this research, we built an indirect laser projection transfer method. First, we sputter requiring material on Si wafer as donor chip. Followed, placed the donor chip upside down on acceptor substrate, and radiated by laser on the donor chip backside. The deposited component on donor substrate would evaporate, and the vapor would fill between two substrates interspacing. Then, the vapor will recrystallize on the cooler acceptor surface. We have successfully transferred low melting temperature 2D materials such as SnSex, NiSex, SbSex on different acceptor substrate, even on flexible and flammable paper. By controlling laser intensity and irradiation time, we can control the amount of Se scattering, finally manipulate SnSe/SnSe2 phase ratio successfully. The mechanism has been demonstrated by SEM, RAMAN, TEM, XPS, XRD. Owing to indirection irradation, it can transfer materials on several fragile substrates without any heat budget, which reveals the great potential of this method for electronics applications. In this report we successfully demonstrate transferred SnSe/SnSe2 have good feasibility in energy storage system and sensor device.

Exciton ionization induced by intersubband absorption in nonpolar ZnO-ZnMgO quantum wells at room temperature

We present a systematic investigation of intersubband transitions in nonintentionally doped $m\text{\ensuremath{-}}\mathrm{plane}$ ZnO/ZnMgO quantum wells (QWs). The investigation is performed using photoinduced absorption spectroscopy at room temperature under optical pumping by a UV laser to generate electron-hole pairs. All samples exhibit TM-polarized intersubbandlike absorption resonances. However, the peak transition energy is largely blueshifted (>100 meV) with expectations from electronic quantum confinement simulations. Based on calculations of the exciton binding energies, we attribute the photoinduced absorption at room temperature to the dissociation of $h{h}_{1}\text{\ensuremath{-}}{e}_{1}$ excitons towards free carriers in the ${e}_{2}$ state and not to $h{h}_{1}\text{\ensuremath{-}}{e}_{1}$ to $h{h}_{1}\text{\ensuremath{-}}{e}_{2}$ excitonic transitions induced by the intersubband absorption as previously stated by Olszakier et al. [Phys. Rev. Lett. 62, 2997 (1989)]. This effect is a consequence of the huge binding energy of excitons in the ZnO material system, which is further enhanced in QWs due to the quantum confinement. This may pave the way for a better understanding of semiconductors' excitonic processes as well as for developing intersubband devices with a blueshifted operating range.

First-Principles Prediction of Superconductivity in High-Buckled Two-Dimensional Tin

Two-dimensional (2D) superconductors exhibit a variety of fascinating phenomena such as the Kosterlitz–Thouless–Berezinskii transition, electron quantum confinement effect, charge density wave, and topologically non-trivial band. Until now, the superconductivity of a classical elemental 2D system of tin has not yet been confirmed. Herein, using the microscopic theory of Bardeen, Cooper, and Schrieffer (BCS) and density functional theory calculations, we meticulously study the electron–phonon coupling, electronic structures, and phase transition of low-buckled (LB) stanene and high-buckled (HB) 2D tin. We demonstrate that the HB 2D tin exhibits remarkable multi-gap electron–phonon coupling superconductivity with critical temperatures approximating those of the conventional bulk tin and few-layer LB stanene. Our results clearly show the strain-induced transitions of the orbital occupation, band structure, and superconductivity from LB to HB structures. Our discovery of HB 2D tin as an excellent 2D superconductor should stimulate the experimental synthesis and characterization of its superconducting properties.

Exciton Binding Energy in CdSe Nanoplatelets Measured by One- and Two-Photon Absorption

Colloidal semiconductor nanoplatelets exhibit strong quantum confinement for electrons and holes as well as excitons in one dimension, while their in-plane motion is free. Because of the large dielectric contrast between the semiconductor and its ligand environment, the Coulomb interaction between electrons and holes is strongly enhanced. By means of one- and two-photon photoluminescence excitation spectroscopy, we measure the energies of the 1S and 1P exciton states in CdSe nanoplatelets with thicknesses varied from 3 up to 7 monolayers. By comparison with calculations, performed in the effective mass approximation with account of the dielectric enhancement, we evaluate exciton binding energies of 195–315 meV, which is about 20 times greater than that in bulk CdSe. Our calculations of the effective Coulomb potential for very thin nanoplatelets are close to the Rytova-Keldysh model, and the exciton binding energies are comparable with the values reported for monolayer-thick transition metal dichalcogenides.

Voltage-Tunable Quantum-Dot Array by Patterned Ge -Nanowire-Based Metal-Oxide-Semiconductor Devices

Semiconductor quantum dots (QDs) are being regarded as the primary unit for a wide range of advanced and emerging technologies including electronics, optoelectronics, photovoltaics and biosensing applications as well as the domain of q-bits based quantum information processing. Such QDs are suitable for several novel device applications for their unique property of confining carriers 3-dimensionally creating discrete quantum states. However, the realization of such QDs in practice exhibits serious challenge regarding their fabrication in array with desired scalability and repeatability as well as control over the quantum states at room temperature. In this context, the current work reports the fabrication of an array of highly scaled Ge-nanowire (radius ~25 nm) based vertical metal-oxide-semiconductor devices that can operate as voltage tunable quantum dots at room temperature. The electrons in such nanowire experience a geometrical confinement in the radial direction, whereas, they can be confined axially by tuning the applied bias in order to manipulate the quantum states. Such quantum confinement of electrons has been confirmed from the step-like responses in the room temperature capacitance-voltage (C-V) characteristics at relatively low frequency (200 kHz). Each of such steps has observed to encompass convolution of the quantized states occupying ~6 electronic charges. The details of such carrier confinement are analyzed in the current work by theoretically modeling the device transport properties based on non-equilibrium Green's function (NEGF) formalism.

Proximity control of interlayer exciton-phonon hybridization in van der Waals heterostructures

Van der Waals stacking has provided unprecedented flexibility in shaping many-body interactions by controlling electronic quantum confinement and orbital overlap. Theory has predicted that also electron-phonon coupling critically influences the quantum ground state of low-dimensional systems. Here we introduce proximity-controlled strong-coupling between Coulomb correlations and lattice dynamics in neighbouring van der Waals materials, creating new electrically neutral hybrid eigenmodes. Specifically, we explore how the internal orbital 1s-2p transition of Coulomb-bound electron-hole pairs in monolayer tungsten diselenide resonantly hybridizes with lattice vibrations of a polar capping layer of gypsum, giving rise to exciton-phonon mixed eigenmodes, called excitonic Lyman polarons. Tuning orbital exciton resonances across the vibrational resonances, we observe distinct anticrossing and polarons with adjustable exciton and phonon compositions. Such proximity-induced hybridization can be further controlled by quantum designing the spatial wavefunction overlap of excitons and phonons, providing a promising new strategy to engineer novel ground states of two-dimensional systems.

Strain-inducing photochemical chlorination of graphene nanoribbons on SiC (0001)

As different low-dimensional materials are sought to be incorporated into microelectronic devices, graphene integration is dependent on the development of band gap opening strategies. Amidst the different methods currently investigated, application of strain and use of electronic quantum confinement have shown promising results. In the present work, epitaxial graphene nanoribbons (GNR), formed by surface graphitization of SiC (0001) on crystalline step edges, were submitted to photochemical chlorination. The incorporation of Cl into the buffer layer underlying graphene increased the compressive uniaxial strain in the ribbons. Such method is a promising tool for tuning the band gap of GNRs.

Morphological characterization and electronic properties of pristine and oxygen-exposed graphene nanoribbons on Ag(110).

Graphene nanoribbons (GNRs) are at the frontier of research on graphene materials since the 1D quantum confinement of electrons allows for the opening of an energy gap. GNRs of uniform and well-defined size and shape can be grown using the bottom-up approach, i.e. by surface assisted polymerization of aromatic hydrocarbons. Since the electronic properties of the nanostructures depend on their width and on their edge states, by careful choice of the precursor molecule it is possible to design GNRs with tailored properties. A key issue for their application in nanoelectronics is their stability under operative conditions. Here, we characterize pristine and oxygen-exposed 1.0 nm wide GNRs with a well-defined mixed edge-site sequence (two zig-zag and one armchair) synthesized on Ag(110) from 1,6-dibromo-pyrene precursors. The energy gap and the presence of quantum confined states are investigated by scanning tunneling spectroscopy. The effect of oxygen exposure under ultra-high vacuum conditions is inferred from scanning tunneling microscopy images and photoemission spectra. Our results demonstrate that oxygen exposure deeply affects the overall system by interacting both with the nanoribbons and with the substrate; this factor must be considered for supported GNRs under operative conditions.

Research on the quantum confinement effect and enhanced luminescence of red-emitting P5+-doped CaAl12O19:Mn4+,Mg2+ phosphors.

Mn4+-activated oxide red phosphors are always a hot topic in the luminescent material field to solve the lack of red light components in white-light-emitting diodes (WLEDs). Herein, a series of novel deep red-emitting CaAl12-mPmO19+m:0.01Mn4+,0.2Mg2+ (m = 0-0.15) phosphors were synthesized and their crystal structure, luminescence properties and thermal stability were investigated in detail. The high-valence P5+ is used to replace low-valence Al3+ in the luminescent host CaAl12O19 to improve the photoluminescence quantum yield (PLQY) of phosphors. The doping of P5+ does not change the crystal phase structure of phosphors, and the luminescence intensity and PLQY are significantly enhanced. The analysis of the photocurrent and fluorescence lifetime shows that an electron trap with a quantum-confinement structure is formed in the phosphor host, which plays a key role in buffering photogenerated electrons. Therefore, the PLQY of the P5+-doped CaAl11.90P0.1O19.10:0.01Mn4+,0.2Mg2+ phosphor increased from 9.8% (P5+-undoped) to 70.2%, and the mechanism of PLQY enhancement is proposed based on the analysis of the crystal structure. Furthermore, the phosphor has superior thermal stability and color purity (96.8%). Overall, this work provides new insights and ideas on quantum confinement effects for improving the quantum yield of Mn4+-activated luminescent materials.