Quantum-confined Structures Research Articles

Alkali-metal-based molecular-ion secondary ion mass spectrometry for precise quantitative analysis of low-dimensional materials without standards

Advancement in the understanding of fundamental aspects of Secondary Ion Mass Spectrometry (SIMS) has made this technique to be extremely powerful for quantitative materials analysis. The secondary ion emission process is a complex phenomenon involving various mechanisms to understand the mechanism. Among the existing mechanisms, the “electron-tunneling model” is the most accepted concept in the understanding of ionization probability for positive and negative ions. This model essentially concerns the survival probability of an ion emitted from the surface. As the intensity of secondary ions of a certain element depends strongly on the efficiency of ionization of a sputtered atom or molecule, the “local surface chemistry” plays a significant role in the secondary ion emission. This is purported as the “Matrix Effect” in SIMS, making the technique extremely challenging for quantification of materials in spite of the fact that it has the highest detection sensitivity (<ppb) and exceptional depth-resolution (<1[Formula: see text]nm). Therefore, it is required to correct the “matrix effect” for quantitative analysis. If the probing element (M) in its close proximity sees an alkali element (such as Li, Rb, K, Na and Cs, referred to as “A”) on the sample surface, the sputtered neutral atom (M0) of the probing element can attach an alkali-ion forming a quasi-molecular (MA)[Formula: see text] ion. Such a phenomenon can also occur if an alkali ion beam is chosen as the primary beam for sputtering. The MA[Formula: see text] molecular ion formation is directly related to the atomic polarizability of the element M. Since the sputtering of neutral atom (M0) is unconnected with the formation of MA[Formula: see text] ion, the “matrix effect” decreases dramatically in the analysis using MA[Formula: see text] ions. The phenomenon is very similar to the mechanism of “secondary neutral mass spectrometry” (SNMS). Although these (MA)[Formula: see text] molecular ions are potentially applicable in the direct quantification of materials without the need for “calibration standards”, it generally has a weak intensity. In such a case, the (MA)[Formula: see text] molecular ions are preferred in the analysis because of their higher detection sensitivity (by a few orders of magnitude in some cases), compared to that of (MA)[Formula: see text] molecular ions. Monitoring of the molecular ions is routinely preferred in conventional SIMS experiments to discriminate the sputtered ion species which are affected by mass interference or have poor dynamic ranges. For example, while making the SIMS analysis of GaAs, carbon as an impurity element is detected by monitoring (AsC)[Formula: see text] molecular ions instead of C− ions, because the C− ions have high background arising from the carbon-containing residual-gas species in the analysis chamber. Caesium is decidedly chosen for MCs[Formula: see text] or MCs[Formula: see text] molecular ions in SIMS because of the strongest reactivity and electropositive nature of caesium. This paper deals with a comprehensive discussion on the ion-emission phenomena in sputtering, the “matrix effect” and its compensation, and the potential application of the “MCs[Formula: see text]-SIMS” ([Formula: see text]) in the effective quantitative chemical analysis of materials. A special prominence has been given to the quantification of low-dimensional materials, superlattices and quantum-confined structures using this state-of-the-art “MCs[Formula: see text]-SIMS” approach in all complexities.

Stimulating Phonon Bottleneck Effect in Organic Semiconductors by Charge-Transfer-Mediated J-Aggregation.

Hot carriers rapidly lose kinetic energies on a subpicosecond time scale, posing significant limitations on semiconductors' photon-conversion efficiencies. To slow the hot carrier cooling, the phonon bottleneck effect is constructed prevalently in quantum-confined structures with discrete energy levels. However, the maximum energy separation (ΔEES) between the energy levels is in a range of several hundred meV, leading to unsatisfactory cooling time. To address this, we design a novel organic semiconductor capable of forming intermolecular charge transfer (CT) in J-aggregates, where the lowest singlet excited state (S1) splits into two states due to the significant interplay between the Coulomb interaction and intermolecular CT coupling. The ΔEES between the two states can be adjusted up to 1.02 eV, and an extremely slow carrier cooling process of ∼72.3 ps was observed by femtosecond transient absorption spectroscopy. Moreover, the phonon bottleneck effect was identified in organic materials for the first time, and CT-mediated J-aggregation with short-range interactions was found to be the key to achieving large ΔEES. The significantly prolonged carrier cooling time, compared to <100 fs in the isolated molecule (10-6 M), highlights the potential of organic molecules with diversified aggregation structures in achieving long-lived hot carriers. These findings provide valuable insights into the intrinsic photophysics of electron-phonon scattering in organic semiconductors.

Charge Oscillations in Bilayer Graphene Quantum Confinement Devices.

Quantum confinement structures are building blocks of quantum devices in fundamental physics exploration and technological applications. In this work, we fabricate dual-gated bilayer graphene Fabry-Pérot quantum Hall interferometers employing two different gating strategies and conduct finite element simulations to understand the electrostatics of the confinement structures and to guide device design and fabrication. We observe two types of resistance oscillations arising from the charging of quantum dots formed inside the interferometers. We obtain the size, location, and charging energy of the dots by measuring the dependence of the oscillations on the magnetic field, gate voltages, and dc bias. We analyze and discuss the origin of the quantum dots and their impact on quantum Hall edge state backscattering and interference. Insights gained in these studies shed light on the construction of van der Waals quantum confinement devices.

Efficient 2D to 0D Energy Transfer in HgTe Nanoplatelet-Quantum Dot Heterostructures through High-Speed Exciton Diffusion.

Large area absorbers with localized defect emission are of interest for energy concentration via the antenna effect. Transfer between 2D and 0D quantum-confined structures is advantageous as it affords maximal lateral area antennas with continuously tunable emission. We report the quantum efficiency of energy transfer in in situ grown HgTe nanoplatelet (NPL)/quantum dot (QD) heterostructures to be near unity (>85%), while energy transfer in separately synthesized and well separated solutions of HgTe NPLs to QDs only reaches 47 ± 11% at considerably higher QD concentrations. Using Kinetic Monte Carlo simulations, we estimate an exciton diffusion constant of 1-10 cm2/s in HgTe NPLs, the same magnitude as that of 2D semiconductors. We also simulate in-solution energy transfer between NPLs and QDs, recovering an R-4 dependence consistent with 2D-0D near-field energy transfer even in randomly distributed NPL/QD mixtures. This highlights the advantage of NPLs 2D morphology and the efficiency of NPL/QD heterostructures and mixtures for energy harvesting.

Directly-coupled well-wire hybrid quantum confinement lasers with the enhanced high temperature performance

It is well known that the laser diode performance will inevitably deteriorate when the device is heated. It has been a difficult issue to solve to date. In this letter, we are reporting a new solution to improve high-temperature performance of the laser diodes. The device uses a kind of directly-coupled well-wire hybrid quantum confinement (HQC) structure of the active medium based on the InGaAs–GaAs–GaAsP material system. This special HQC structure is constructed based on the strain-driven indium (In)-segregation effect and the growth orientation-dependent on-GaAs multi-atomic step effect. The measurement and analysis for the HQC laser sample show that the carrier leakage loss, the Auger recombination and gain-peak shifting due to heating are reduced in the HQC structure. It therefore increases the optical gain for lasing at high temperature. The power conversion efficiency is enhanced by >57% and the threshold carrier density drops by >24% at T ⩾ 360 K, in comparison to the traditional quantum-well laser performance. A higher characteristic temperature of 240 K is obtained as well. It implies the better thermal stability of the HQC laser structure. These achievements show a significant prospect for developing high thermo-optic performance of laser diodes.

Gamma-Ray Irradiation Induced Ultrahigh Room-Temperature Ferromagnetism in MoS2 Sputtered Few-Layered Thin Films

Defect engineering is of great interest to the two-dimensional (2D) materials community. If nonmagnetic transition-metal dichalcogenides can possess room-temperature ferromagnetism (RTFM) induced by defects, then they will be ideal for application as spintronic materials and also for studying the relation between electronic and magnetic properties of quantum-confined structures. Thus, in this work, we aimed to study gamma-ray irradiation effects on MoS2, which is diamagnetic in nature. We found that gamma-ray exposure up to 9 kGy on few-layered (3.5 nm) MoS2 films induces an ultrahigh saturation magnetization of around 610 emu/cm3 at RT, whereas no significant changes were observed in the structure and magnetism of bulk MoS2 (40 nm) films even after gamma-ray irradiation. The RTFM in a few-layered gamma-ray irradiated sample is most likely due to the bound magnetic polaron created by the spin interaction of Mo 4d ions with trapped electrons present at sulfur vacancies. In addition, density functional theory (DFT) calculations suggest that the defect containing one Mo and two S vacancies is the dominant defect inducing the RTFM in MoS2. These DFT results are consistent with Raman, X-ray photoelectron spectroscopy, and ESR spectroscopy results, and they confirm the breakage of Mo and S bonds and the existence of vacancies after gamma-ray irradiation. Overall, this study suggests that the occurrence of magnetism in gamma-ray irradiated MoS2 few-layered films could be attributed to the synergistic effects of magnetic moments arising from the existence of both Mo and S vacancies as well as lattice distortion of the MoS2 structure.

Two-Photon Lasing from Two-Dimensional Homologous Ruddlesden-Popper Perovskite with Giant Nonlinear Absorption and Natural Microcavities.

Two-dimensional Ruddlesden-Popper perovskites (RPPs) with multiple quantum well-like structures, strong excitonic quantum confinement, and high stability are promising optical gain media. However, the lasing from such material with a small number of inorganic well layers is difficult to achieve. Herein, we demonstrate the low-threshold upconversion lasing from the homologous RPP (PEA)2(MA)n-1PbnI3n+1 (n = 2 and 3) microflakes with wavelength varies from 598 to 637 nm under 800 nm laser excitation at low temperature (≤153 K). Using the micro Z-scan technique, we discovered that the RPP flakes have a giant two-photon absorption coefficient β as high as 3.6 × 103 cm GW-1, resulting in the effective upconversion transition under two-photon excitation. Furthermore, the self-formation of Fabry-Pérot microcavities provides the support for lasing emission from the n ≥ 2 RPP flakes. Calculation results and microscopic transient absorption measurements reveal that low-threshold lasing is due to the high differential gain coefficient and the suppressed nonradiative Auger recombination rate inside the quantum confinement structures. These properties enable RPPs as potential gain media for developing upconversion microcavity lasers.

Optimization of photonic crystal structure by FDTD method to improve the light extraction efficiency in silicon

Silicon photonics technology is on its last hurdle to complete the system with active research on enhancing the light extraction efficiency of silicon. Silicon has a very poor radiative emission efficiency due to its indirect bandgap, thus making the probability of radiative recombination to be very low. The periodic quantum confinement structures such as photonic crystals (PhCs) can be tailored to increase the radiative recombination probability at the desired wavelengths. In this paper, we have optimized the silicon photonic crystal (Si PhCs) parameters to obtain luminescence at silicon bandgap and telecommunication wavelength using FDTD method. The extraction of photons is mostly conquered in the silicon structure that has lattice constant, a: 480 nm with radius, r: 140 nm and a: 500 nm with r: 160 nm. We have also observed the emissive photon luminescence that emerges in the most stable state of optical communication wavelength at 1310 nm.

Strongly Quantum-Confined Perovskite Nanowire Arrays for Color-Tunable Blue-Light-Emitting Diodes.

Color tunability of perovskite light-emitting diodes (PeLEDs) by mixed halide compositional engineering is one of the primary intriguing characteristics of PeLEDs. However, mixed halide PeLEDs are often susceptible to color red-shifting caused by halide ion segregation. In this work, strongly quantum-confined perovskite nanowires (QPNWs) made of CsPbBr3 are grown in nanoporous anodic alumina templates using a closed space sublimation process. By tuning the pore size with atomic layer deposition, QPNWs with a diameter of 6.6 to 2.8 nm have been successfully obtained, with continuous tunable photoluminescence emission color from green (512 nm) to pure blue (467 nm). To better understand the photophysics of QPNWs, carrier dynamics and the benefit of alumina passivation are studied and discussed in detail. Eventually, PeLEDs using various diameters of CsPbBr3 QPNWs are successfully fabricated with cyan color (492 nm) PeLEDs, achieving a record high 7.1% external quantum efficiency (EQE) for all CsPbBr3-based cyan color PeLEDs. Sky blue (481 nm) and pure blue (467 nm) PeLEDs have also been successfully demonstrated, respectively. The work here demonstrates a different approach to achieve quantum-confined one-dimensional perovskite structures and color-tunable PeLEDs, particularly blue PeLEDs.

Atomically Resolved Quantum-Confined Electronic Structures at Organic-Inorganic Interfaces of Two-Dimensional Ruddlesden-Popper Halide Perovskites.

This work demonstrates the direct visualization of atomically resolved quantum-confined electronic structures at organic-inorganic heterointerfaces of two-dimensional (2D) organic-inorganic hybrid Ruddlesden-Popper perovskites (RPPs); this is accomplished with scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) by using solvent engineering to prepare perpendicularly oriented 2D RPPs. Atomically resolved band mapping images across the organic-inorganic interfaces of 2D RPPs yield typical quantum-well-like type-I heterojunction band alignment with band gaps depending on the thicknesses or n values of the inorganic perovskite slabs. The presence of edge states within the band gap due to organic cation vacancies is also observed. In addition, real-space visualization of atomic-scale structural phase transition behavior and changes in local electronic band structures are obtained simultaneously. Our results provide an unequivocal observation and explanation of the quantum-confined electronic structures formed at organic-inorganic interfaces of 2D RPPs.

Signature of Spin-Resolved Quantum Point Contact in p-Type Trilayer WSe2 van der Waals Heterostructure.

In this study, an electrostatically induced quantum confinement structure, so-called quantum point contact, has been realized in a p-type trilayer tungsten diselenide-based van der Waals heterostructure with modified van der Waals contact method with degenerately doped transition metal dichalcogenide crystals. Clear quantized conductance and pinch-off state through the one-dimensional confinement were observed by dual-gating of split gate electrodes and top gate. Conductance plateaus were observed at a step of e2/h in addition to quarter plateaus such as 0.25 × 2e2/h at a finite bias voltage condition indicating the signature of intrinsic spin-polarized quantum point contact.

Carrier Recovery from Sub-Bandgap States in a GaN-Based Quantum-Confined Structure: Identification of Carrier Reservoirs through Femtosecond Pump-Probe Spectroscopy

GaN-based quantum-confined structures with an optical cavity are frequently used for solid-state light sources. The confined active and barrier regions of the structures consist of low- and high-ba...

Temperature Dependence of Carrier Extraction Processes in GaSb/AlGaAs Quantum Nanostructure Intermediate-Band Solar Cells.

From the viewpoint of band engineering, the use of GaSb quantum nanostructures is expected to lead to highly efficient intermediate-band solar cells (IBSCs). In IBSCs, current generation via two-step optical excitations through the intermediate band is the key to the operating principle. This mechanism requires the formation of a strong quantum confinement structure. Therefore, we focused on the material system with GaSb quantum nanostructures embedded in AlGaAs layers. However, studies involving crystal growth of GaSb quantum nanostructures on AlGaAs layers have rarely been reported. In our work, we fabricated GaSb quantum dots (QDs) and quantum rings (QRs) on AlGaAs layers via molecular-beam epitaxy. Using the Stranski–Krastanov growth mode, we demonstrated that lens-shaped GaSb QDs can be fabricated on AlGaAs layers. In addition, atomic force microscopy measurements revealed that GaSb QDs could be changed to QRs under irradiation with an As molecular beam even when they were deposited onto AlGaAs layers. We also investigated the suitability of GaSb/AlGaAs QDSCs and QRSCs for use in IBSCs by evaluating the temperature characteristics of their external quantum efficiency. For the GaSb/AlGaAs material system, the QDSC was found to have slightly better two-step optical excitation temperature characteristics than the QRSC.

(Invited) Non-Equilibrium Treatment of Molecular Dopants for 2D Materials

2D materials gather attention due to their unique and quantum-confined electronic structures applicable to optoelectronics and a platform for physics. One representative 2D material is transition metal dichalcogenides (TMDCs), which consist of a three-atomic-layered structure with a thickness of only ~0.7 nm. Representative TMDCs are MoS2 and WSe2, and both materials are semiconducting materials with the direct-band gap at the monolayer limit. To apply these unique materials for atomically thin electronics and quantum devices, controlling the carrier concentration is important. In this talk, I will present our recent results to control the carrier behaviors of TMDCs using molecules, with a particular focus on the molecules in non-equilibrium environments, starting with the preparation process of a dopant molecule in one (Ref.1). We found a method to prepare a strong electron dopant, benzyl viologen (BV), in a non-equilibrium manner. In the previous works, the BV molecule is synthesized using a strong reducing agent, sodium borohydride (NaBH4), in water. However, the method is inefficient because NaBH4 reacts vigorously with water at the same time. We replaced NaBH4 with indium metal and found a synthetic method to prepare BV dopant molecules under a non-equilibrium convection flow environment. The convection flow continuously synthesizes BV dopant molecules to a high concentration; furthermore, the method eliminates the purification process to use the dopant solution. Additionally, I will show a non-equilibrium dopant molecular assembly on TMDCs (Ref. 2 and 3). BV molecules usually interact immediately with the TMDC surface and assemble uniformly. Because of the hydrophobic character of the BV molecules, it is difficult to apply photoresists to make local carrier modulation at submicron scales. We overcome the issue by using the self-pattern formation process of the BV molecular films on the surface of TMDCs. By applying hydrophobic/hydrophilic repulsive interaction, the BV film destabilizes and shows pattern formation at scales of about ~200 nm periodicity. Our analysis found that the patterns induce pn and metal/semiconductor junctions on TMDCs. This non-equilibrium process would be useful to prepare electronic device structures via the spontaneous process, which is uniformly distributed at large scale. Keywords: Molecular dopant, Transition metal dichalcogenides, TMDCs, convection flow, molecular assembly, spontaneous pattern formation

Multidimensional Perovskite Cesium Lead Halide Nanocrystal for Light Conversion Applications

Zero-dimensional (0D) inorganic perovskites have recently emerged as a new class of material for optoelectronics owing to their outstanding excitonic properties, strong photoluminescence (PL), and high exciton binding energy. These materials have a unique quantum-confined structure, which originates from the presence of fully isolated octahedra exhibiting single-molecule behavior. In this work, we probed the optical behavior of single molecule-like isolated octahedra in 0D Cs4PbX6 (X = Cl, Br/Cl) perovskite through isovalent (Mn2+) doping at Pb sites. The incorporation of Mn2+ stabilizes the Cs4PbX6 phase by lowering the symmetry of PbX6 via enhanced octahedral distortion (preventing the high-symmetry cubic CsPbX3 impurity) and controlling the compositional variation of Cs-Pb salts. This strategy enabled the synthesis of CsPbX3 free Cs4PbX6 nanocrystals. A high PL quantum yield (QY) of Mn2+ emission was obtained in the colloidal (29%) and solid (21%, powder) forms. These performances can be attributed to structure-induced confinement effects, which enhance the energy transfer from localized host exciton states to Mn2+ dopant within the isolated octahedra of 0D perovskite. The present findings could lead to designing of low-dimensional perovskites as efficient light emitters with photo and chemical stability for high-performance optoelectronic devices.

Long-range electron-hole exchange interaction in aluminum nitride

To resolve the discrepancies in the exciton fine structure of aluminum nitride (AlN), polarization- and angle-resolved photoluminescence (PL) spectroscopies are performed. The excitonic PL spectra strongly depend on the optical polarization and detection angle. We propose that both the long-range and short-range electron-hole exchange interaction should be used to interpret the luminescence spectra. The theoretical framework fully explains the present and previous experimental results. The large longitudinal-transverse splitting energy obtained in this study suggests that AlN has strong light-matter coupling without quantum-confined structures.

Ultrabroadband and independent polarization of optical amplification with InGaAs-based indium-rich cluster quantum-confined structure

This Letter reports polarization-independent optical amplification over an ultrabroad spectral range by semiconductor optical amplifiers. The technique uses an InGaAs-based indium-rich cluster (IRC) quantum-confined structure as the active medium and obtains comparable optical gain for both transverse electric (TE) and transverse magnetic (TM) polarization modes in the spectral ranges of 905–1005 and 905–970 nm, respectively. The device thus provides independent optical amplification for TE and TM polarizations over a common bandwidth of 65 nm. The difference between the amplified intensities of TE and TM modes is &amp;lt;0.5 dB. These results are attributed to the special emission mechanism of the IRC quantum-confined structure, which differs from that of conventional quantum wells or quantum dots. A preliminary analysis of this mechanism is provided.

Multilayered 2D Cesium‐Based Hybrid Perovskite with Strong Polarization Sensitivity: Dimensional Reduction of CsPbBr3

Three-dimensional (3D) perovskite CsPbBr 3 has recently taken a blooming position for optoelectronic applications. However, due to the lack of natural anisotropy of optical attributes, it is a great challenge to fulfil polarization-sensitive photodetection. Here, for the first time, we exploited dimensionality reduction of CsPbBr 3 to tailor 2D multilayered hybrid perovskite, (TRA) 2 CsPb 2 Br 7 (1, where TRA is (carboxy)cyclohexylmethylammonium), serving as a potential polarized-light detecting candidate. Its unique quantum-confined 2D structure results in intrinsic anisotropy of electrical conductivity, optical absorbance and polarization-dependent responses. Particularly, it exhibits remarkable dichroism with the photocurrent ratio ( I pc / I pa ) of ~ 2.1, being much higher than that of isotropic CsPbBr 3 crystal and reported CH 3 NH 3 PbI 3 nanowire ( ~ 1.3), which reveals its great potentials for polarization-sensitive photodetection. Further, crystal-based detectors of 1 show fascinating responses to the polarized-light, including high detectivity (>10 10 Jones), fast responding time ( ~ 300μs) and sizeable on/off current ratios (>10 4 ). To our best knowledge, this is the first study on 2D Cs-based hybrid perovskite exhibiting strong polarization-sensitivity. The work highlights an effective pathway to explore new polarization-sensitive candidates for hybrid perovskites and promotes their future electronic applications.

Josephson Effect and Charge Distribution in Thin Bi2Te3 Topological Insulators

Thin layers of topological insulator materials are quasi-2D systems featuring a complex interplay between quantum confinement and topological band structure. To understand the role of the spatial distribution of carriers in electrical transport, the Josephson effect, magnetotransport, and weak anti-localization are studied in bottom-gated thin Bi2 Te3 topological insulator films. The experimental carrier densities are compared to a model based on the solutions of the self-consistent Schrödinger-Poisson equations and they are in excellent agreement. The modeling allows for a quantitative interpretation of the weak antilocalization correction to the conduction and of the critical current of Josephson junctions with weak links made from such films without any ad hoc assumptions.

Dimensional Reduction of Cs2 AgBiBr6 : A 2D Hybrid Double Perovskite with Strong Polarization Sensitivity.

By dimensional reduction of the 3D motif of Cs2 AgBiBr6 , a lead-free 2D hybrid double perovskite, (i-PA)2 CsAgBiBr7 (1, i-PA=isopentylammonium), was successfully designed. It adopts a quantum-confined bilayered structure with alternating organic and inorganic sheets. Strikingly, the unique 2D architecture endows it highly anisotropic nature of physical properties, including electric conductivity and optical absorption (the ratio αb /αc =1.9 at 405 nm). Such anisotropy attributes result in the strong polarization-sensitive responses with large dichroic ratios up to 1.35, being comparable to some 2D inorganic materials. This is the first study on the hybrid double perovskites with strong polarization sensitivity. A crystal device of 1 also exhibits rapid response speed (ca. 200 μs) and excellent stabilities. The family of 2D hybrid double perovskites are promising optoelectronic candidates, and this work paves a new pathway for exploring new green polarization-sensitive materials.