II-VI Semiconductor Research Articles

Exciton Fine Structure in Axially Symmetric Quantum Dots and Rods of III-V and II-VI Semiconductors.

Both absorption and emission of light in semiconductor quantum dots occur through excitation or recombination of confined electron-hole pairs, or excitons, with tunable size-dependent resonant frequencies that are ideal for applications in various fields. Some of these applications require control over quantum dot shape uniformity, while for others, control over energy splittings among exciton states emitting light in different polarizations and/or between bright and dark exciton states is of key importance. These splittings, known as exciton fine structure, are very sensitive to the nanocrystal shape. Theoretically, nanocrystals of spheroidal shape are often considered, and their nonsphericity is treated perturbatively as stemming from a linear uniaxial deformation of a sphere. Here, we compare this treatment with a nonperturbative model of a cylindrical box, free of any restrictions on the cylinder's aspect ratio. This comparison allows one to understand the limits of validity of the traditional perturbative model and offers insights into the relative importance of various mechanisms controlling the exciton fine structure. These insights are relevant to both colloidal nanocrystals and epitaxial quantum dots of III-V and II-VI semiconductors.

Self-Assembly of Chiral Ligands on 2D Semiconductor Nanoplatelets for High Circular Dichroism.

Group II-VI semiconductor nanoplatelets (NPLs) with atomically defined thicknesses and extended atomically flat (001) facets are used for ligand binding and chiro-optical effects. In this study, we demonstrate that tartrate ligands, anchored by two carboxylate groups, chelate the (001) facets of NPLs at an average ratio of one tartrate molecule to two cadmium (Cd) surface atoms. This assembly of chiral molecules on inorganic nanocrystals generates a circular dichroism g-factor as high as 1.3 × 10-2 at the first excitonic transition wavelength of NPLs. Tartrate ligands induce an orthorhombic distortion of the initially "cubic" crystal structure, classifying the NPLs within the 222-point group. Unlike spherical nanocrystals, where it is difficult to discern whether chiral ligands affect only the surface atoms or the entire crystal structure, our findings unequivocally show that the crystal structure of NPLs is modified due to their thinness and atomically precise thickness. The in-plane lattice parameters experience compressive and tensile stresses, significantly splitting the heavy-hole and light-hole bands. Additionally, tartrate ligands adopt different conformations on the NPL surface over time, resulting in dynamic changes in the circular dichroism signal, including an inversion of its sign.

Black Phosphorus for Mid-Infrared Optoelectronics: Photophysics, Scalable Processing, and Device Applications.

High efficiency mid-infrared (λ = 3-8 μm) light emitters and photodetectors are pivotal for advancing next-generation optoelectronics. However, narrow-bandgap semiconductors face fundamental challenges such as pronounced nonradiative carrier recombination and thermally generated noise, which impede device performance. Recently, two-dimensional layered black phosphorus (BP) and its alloys have attracted substantial interest for mid-infrared device applications, demonstrating superior performance relative to conventional III-V and II-VI semiconductors with similar bandgaps. In this review, we discuss the optical properties of BP, contrasting these with those of covalent compounds. Owing to its inherently self-terminated surface structure and reduced nonradiative recombination, BP exhibits high performance in light emission and photodetection at room temperature. Furthermore, this review highlights recent advances in the large-area processing of BP thin films, paving the way for practical device applications and integration. Finally, we explore ongoing challenges and emerging opportunities in the utilization of BP for functional mid-infrared devices.

Ion exchange in semiconductor magic-size clusters.

As a crucial post-synthesis method, ion exchange allows for precise control over the composition, interface, and morphology of nanocrystals at the atomic scale, achieving material properties that are difficult to obtain with traditional synthesis techniques. In nanomaterial science, semiconductor magic-size clusters (MSCs), with their atomic-level precision and unique quantum confinement effects, serve as a bridge between molecules and nanocrystals. Despite this, research on ion exchange in MSCs is still in its infancy. This review introduces the principles of ion exchange and reactions in colloidal nanocrystals and MSCs, analyzing the importance and challenges of ion exchange in studying MSCs. This paper begins with a focus on the current research progress of cation and anion exchange in II-VI and III-V semiconductor MSCs. Then, the common methods for characterizing MSCs during the ion exchange process are discussed. Finally, the article envisions future research directions based on MSCs' ion exchange. Research on MSCs' ion exchange not only aids in designing MSCs with complex functionalities, but also plays an essential role in elucidating the ion exchange mechanisms in nanocrystals, providing new insights for the innovative design and synthesis of nanomaterials.

Syntheses, Properties, and Applications of ZnS-Based Nanomaterials

ZnS is a II-VI semiconductor with a wide bandgap. ZnS-based nanomaterials have been produced in a variety of morphologies with unique properties and characteristic features. An extensive collection of research activities is available on various synthetic methodologies to produce such a wide variety of ZnS-based nanomaterials. In this comprehensive review, we thoroughly covered all the different synthetic techniques employed by researchers across the globe to produce zero-dimensional, one-dimensional, two-dimensional, and three-dimensional ZnS-based nanomaterials. Depending on their morphologies and properties, ZnS-based nanomaterials have found many applications, including optoelectronics, sensors, catalysts, batteries, solar cells, and biomedical fields. The properties and applications of ZnS-based nanostructures are described, and the scope of the future direction is highlighted.

The effect of quantum confinement and the role of electron-phonon interaction on the band gap shrinkage of some II-VI semiconductors

The role of electron-phonon interaction in band gap shrinkage for some II-VI bulk and low-dimensional semiconductors is investigated in this work. The variation of the energy band gap is studied as a function of temperature using Varshni's, Vina's, and Passler's relations. It is observed that the change in the energy band gap is affected due to the quantum confinement as the dimensionality of these semiconductors is decreased.

(Invited) Dynamics of Photoluminescence in Mn-Doped Pb-Halide Perovskite Nanocrystals

Nanocrystals of lead halide perovskites have been shown to have extraordinary photoluminescence properties, spanning the entire visible range achieved by the composition tuning via solid solutions at the halide site. Taking cues from Mn doping of traditional Group II-VI semiconductor quantum dots, Mn doping of the lead halide perovskites has been extensively explored by the community, establishing intense Mn-dopant induced emission that is significantly Stokes shifted from the excitonic emissions of the host. While all reports agree on the essential features of this emission, such as the peak position and the width of the Mn photoluminescence, there appears to be little agreement on the microscopic processes by which the host sensitizes Mn following its photoexcitation and the steps that control the deexcitation leading to the Mn emission. There are even reports of disagreement between different reports on the temperature dependence of the Mn emission and its origin. I shall discuss the present status of this field and establish, with the help of new experimental data as well as the already reported ones, what appears to be the microscopic processes controlling the excited dynamics in the Mn-doped lead halide perovskites.

Towards Large-Area Black Phosphorous Midwave-IR Optoelectronics

High-efficiency mid-wavelength infrared (mid-IR, 3–5 µm) optoelectronics, including light emitting diodes (LEDs) and photodetectors, are of high demand for emerging applications in spectroscopy, imaging, and gas sensing. Black phosphorus (bP) has emerged as a unique optoelectronic material for mid-IR applications with performances surpassing those of conventional III-V and II-VI semiconductors of similar bandgap. In this talk, I will present recent advancements on understanding and controlling the radiative and non-radiative recombination rates as a function of bP thickness. We observe higher photoluminescence quantum yields in bP as compared to conventional III-V and II-VI semiconductors of similar bandgaps due to the smaller Auger recombination rate. As a result, bP mid-IR LEDs with external quantum efficiency of >4% are reported, outperforming the state-of-the-art in this wavelength range. Furthermore, device encapsulation and packaging technologies are explored with extrapolated half lifetime of ~15,000 hours based on accelerated lifetime measurements. Finally, I will present strategies for large-area device fabrication and processing.

(Invited) Transformation Mechanism of Quantum-Sized Semiconductor Nanocrystals Revealed by in Situ Transmission Electron Microscopy

Semiconductor nanocrystal quantum dots are renowned for their remarkable properties, including tunable bandgap, pure color emission with exceptionally narrow bandwidths, and high luminescence efficiency. A thorough understanding of the transformation mechanism of quantum-sized semiconductor nanocrystals is essential for their practical application, as their properties are determined by their structure. This presentation will focus on exploring these mechanisms using in-situ transmission electron microscopy (TEM). The first part of the talk will examine the moisture-induced degradation of quantum-sized semiconductor nanocrystals, highlighting a notable finding: the formation of amorphous intermediates on the surfaces of these nanocrystals during degradation. This insight is crucial for developing strategies to enhance the stability and durability of nanocrystals, thereby extending their practical application. Next, I will discuss the crystal phase transition mechanism in II-VI semiconductor nanocrystals, triggered by off-stoichiometry. This discussion will shed light on the determinants of the crystal phase in these nanocrystals, a key to understanding their functionality. Lastly, the presentation introduces the application of in-situ X-ray scattering. This technique offers structural insights that complement those obtained from in-situ TEM, significantly enriching our understanding of the structural dynamics in semiconductor nanocrystals.

II-VI Semiconductor-Based Conductometric Gas Sensors: Is There a Future for These Sensors?

A review of the state of research in the development of conductometric gas sensors based on II-VI semiconductors is given. It was shown that II-VI compounds indeed have properties that are necessary for the development of highly efficient gas sensors. In this case, to achieve the required parameters, all approaches developed for metal oxides can be used. At the same time, during a detailed review, it was concluded that sensors based on II-VI compounds have no prospects for appearing on the gas sensor market. The main obstacle is the instability of the surface state, which leads to poor reproducibility of parameters and drift of sensor characteristics during operation.

Recent Progress in the Composites of Perovskite Nanocrystals and II-VI Quantum Dots: Their Synthesis, Applications, and Prospects

Abstract: The remarkable photoelectric characteristics of perovskite nanocrystals (NCs), including high fault tolerance, tunable photoluminescence (PL) emission, and high carrier mobility, contribute to making them especially attractive for photonic and optoelectronic applications. Unfortunately, the poor environmental thermal and light stability set obstacles to their industrial applications. Over the past 40 years, II-VI semiconductor quantum dots (QDs) have achieved many important photophysics findings and optoelectronic applications. Compared with perovskite NCs, II-VI semiconductor QDs still have a relatively weaker molar absorbance coefficient. Whereas, significant enhancement of both the stability and the optical performance of the composites of perovskite NCs and II-VI QDs are of interest for photovoltaic and optoelectronic devices. The composites of perovskite NCs and II-VI QDs come in two primary types: core/shell structures and heterojunction structures. To better understand the composites of perovskite NCs and II-VI QDs, the approaches of synthesis methods, their optoelectronic properties, carrier dynamics and potential applications in solar cells, light emitting diodes (LEDs) and photodetectors are summarized. Furthermore, the unmet problems and the potential applications are also presented.

Anisotropic Growth of Covalent Inorganic Complexes to Nanoplatelets.

Colloidal II-VI semiconductor nanoplatelets (NPLs) provide a new platform in material science due to their unique growth mode and advanced optical properties. However, in contrast to the rapid development of zinc blend structured NPLs, studies on the formation of wurtzite (WZ) NPLs have been limited to the lamellar assembly of specific magic-sized nanoclusters (MSCs). Therefore, the study of new precursors is important for enriching the synthesis strategy, improving the study of two-dimensional (2D) nanocrystal growth mechanisms, and constructing complex nanostructures. Here, we demonstrated that covalent inorganic complexes (CICs), as novel functional intermediates, can be directly used to form NPLs without involving MSCs. Using in situ absorption spectra, we demonstrated that the evolution followed a pseudo-first-order kinetics (kobs = 0.02 min-1 (t1/2 = 34.7 min)). Several types of binary WZ NPLs, including CdSe, CdS, CdTe, and ZnS, have been directly prepared based on this mechanism through the anisotropic growth of CICs. In addition, CICs can also be used to prepare Mn-doped CdSe NPLs. The present study not only affords new precursors for the synthesis of WZ NPLs but also advances our understanding of the synthesis mechanism of nanocrystals.

Optical Characterization of Ferric-Doped ZnO Thin Film Deposited By Chemical Bath Deposition Method

Fe-doped ZnO holds numerous potential uses in devices such as UV light emitters, piezoelectric transducers, transparent electronics in solar cells, and bright windows as an II-VI semiconductor material. This study looked into the band gap, surface characteristics, optical transmittance, and absorbance of ZnO thin films with Fe doping in different pH conditions such as 8, 9, and 10. The films were prepared using the chemical bath deposition (CBD) method at 350 °C on a glass substrate. The samples were characterized using UV-visible spectroscopy and scanning electron microscopy (SEM). The SEM images of the undoped samples exhibited a lack of cracks, but the doped samples displayed visible cracks due to the presence of Fe in the films. ZnO thin-film surface shape is altered by pH and Fe doping variations. There is a tendency for the films' visual transmission to decrease when the pH rises. The absorbance for all samples increases in the UV and declines in the visible. Fe doping causes the optical band gap Eg to drop. The band gap of both doped and undoped samples reduces when pH rises.

Growth and Electrical Characterization of Hybrid Core/Shell InAs/CdSe Nanowires.

Core-only InAs nanowires (NWs) remain of continuing interest for application in modern optical and electrical devices. In this paper, we utilize the II-VI semiconductor CdSe as a shell for III-V InAs NWs to protect the electron transport channel in the InAs core from surface effects. This unique material configuration offers both a small lattice mismatch between InAs and CdSe and a pronounced electronic confinement in the core with type-I band alignment at the interface between both materials. Under optimized growth conditions, a smooth interface between the core and shell is obtained. Atom probe tomography (APT) measurements confirm substantial diffusion of In into the shell, forming a remote n-type doping of CdSe. Moreover, field-effect transistors (FETs) are fabricated, and the electron transport characteristics in these devices is investigated. Finally, band structure simulations are performed and confirm the presence of an electron transport channel in the InAs core that, at higher gate voltages, extends into the CdSe shell region. These results provide a promising basis toward the application of hybrid III-V/II-VI core/shell nanowires in modern electronics.

Enhanced Spin Polarization from Biaxially Strained Colloidal Quantum Dots.

Electron and hole spin polarization is crucial for quantum dots to be used in spin lasers and quantum information processing. However, the degree of spin polarization in II-VI and III-V semiconductor quantum dots is low because of the degenerated valence band. Here, we increase the light and heavy hole degeneracy by introducing biaxial strain into CdSe-based quantum dots, enabling the degree of spin polarization to be increased from 20% to 50% under photoexcitation. The optical gain threshold measurement further reveals that the increase in polarization helps to reduce the gain threshold.

Guided CdTe Nanowires Integrated into Fast Near-Infrared Photodetectors.

Infrared photodetectors are essential devices for telecommunication and night vision technologies. Two frequently used materials groups for this technology are III-V and II-VI semiconductors, notably, mercury-cadmium-telluride alloys (MCT). However, growing them usually requires expensive substrates that can only be provided on small scales, and their large-scale production as crystalline nanostructures is challenging. In this paper, we present a two-stage process for creating aligned MCT nanowires (NWs). First, we report the growth of planar CdTe nanowires with controlled orientations on flat and faceted sapphire substrates via the vapor-liquid-solid (VLS) mechanism. We utilize this guided growth approach to parallelly integrate the NWs into fast near-infrared photodetectors with characteristic rise and fall times of ∼100 μs at room temperature. An epitaxial effect of the planar growth and the unique structure of the NWs, including size and composition, are suggested to explain the high performance of the devices. In the second stage, we show that cation exchange with mercury can be applied, resulting in a band gap narrowing of up to 55 meV, corresponding to an exchange of 2% Cd with Hg. This work opens new opportunities for creating small, fast, and sensitive infrared detectors with an engineered band gap operating at room temperature.

Recent progress of innovative infrared avalanche photodetectors

Currently, the development trend of infrared detection systems is oriented towards achieving smaller size, reduced weight, lower power consumption, decreased cost, and heightened performance, collectively referred to as SWAP3. In this context, a rapid evolution is witnessed in the realm of next-generation high-sensitivity infrared avalanche photodiodes (APD). In recent years, the increasing demands for detector performance from astronomical, remote sensing, and civilian applications have presented novel challenges for the mechanistic exploration and device design of traditional avalanche photodetectors. This review explores the forefront of infrared APD technology, focusing on the remarkable progress achieved through the utilization of II-VI and III-V semiconductor families and the exploration based on novel low-dimensional materials. The review provides a detailed analysis of the unique properties and performance enhancements exhibited by APDs based on these material systems, shedding light on their potential for revolutionizing infrared sensing and imaging applications, and inspiring further innovation and exploration in the field of infrared photodetection.

Resonant tunneling in a colloidal CdS semiconductor quantum-dot single-electron transistor based on heteroepitaxial-spherical Au/Pt nanogap electrodes.

Semiconductor quantum dots (QDs) have unique discrete energy levels determined by the particle size and material. Therefore, they have potential applications as novel optical and electronic devices. Among those, colloidal group II-VI semiconductor quantum dots stand out for their facile synthesis and band gaps aligned with the visible light spectrum. However, the electrical characterization studies of an individual quantum dot necessitate the size of nanogap electrodes being equal to the size of the quantum dot, which has conventionally been evaluated using techniques such as scanning tunneling microscopy (STM) and nanogaps fabricated by electromigration. The complexity of device fabrication has restricted research in this area. Here, we present a pioneering approach for the electrical characterization of single-QD: heteroepitaxial-spherical (HS) Au/Pt nanogap electrodes. We fabricated transistors through chemisorption, an anchoring colloidal CdS QD (3.8 nm) between the HS-Au/Pt nanogap electrodes (gap separation: 4.5 nm). The resulting device functions as a quantum-dot single-electron transistor (QD-SET), showing resonant tunneling-an inherent characteristic of the QD. A steep current increase was observed at a negative voltage, apart from the theoretical single-electron tunneling current by Coulomb blockade phenomena, which agreed with the theoretical resonant tunneling current through a discrete energy level of the QD. This underscores the promise of HS-Au/Pt nanogap electrodes in realizing single-QD devices, offering a pathway toward unlocking their full potential.

Large-scale efficient mid-wave infrared optoelectronics based on black phosphorus ink

The mid-wave infrared (MWIR), ranging from 2 to 5 micrometers, is of substantial interest for chemical sensing, imaging, and spectroscopy. Black phosphorus (bP)–based MWIR light emitters and detectors have been shown to outperform the state-of-the-art for commercial devices due to the low Auger recombination coefficient of bP. However, the scalability of these devices remains a challenge. Here, we report a bP ink formula that preserves the exceptional MWIR optoelectronic properties of bP to deposit centimeter-scale, uniform, and pinhole free films with a photoluminescence quantum yield higher than competing III-V and II-VI semiconductors with similar bandgaps at high excitation regime. As a proof of concept, we use bP ink as a “phosphor” on a red commercial light-emitting diode to demonstrate bright MWIR light emission. We also show that these films can be integrated into heterostructure device architectures with electron and hole selective contacts for direct-injected light emission and detection in MWIR.

Supernormal enrichment of cadmium in sphalerite via coupled dissolution-reprecipitation process

Supernormal enrichment of cadmium in sphalerite is frequently observed in some sedimentary-hosted zinc-lead deposits, although related fluid process remains unconstrained. At the Jinding Mississippi Valley-type deposit, a considerable portion of cadmium has been remobilized from early sphalerite through coupled dissolution-reprecipitation reaction to form high-grade ores. Here we report natural occurrence of various sulfide nanoparticles and related textures in sphalerite that help document this process. A nanoscale study by transmission electron microscopy provides a rare glimpse of phase transition of cadmium from lattice-bond impurity, composition anomaly along planar defects, aggregates of hexagonal cadmium sulfide nanoparticles, to crystalline greenockite inclusions. Such a process may be mediated by oxidative dissolution on early-formed cadmiferous sphalerite during injection of cadmium-rich oxidative acidic fluids. This study provides an alternative mechanism versus exclusively solid-state diffusion for dispersed elements’ redistribution in hydrothermal ore deposits. It also sheds light on artificial synthesis of II-VI semiconductor nanomaterials by similar methods.