Te Quantum Research Articles

An innovative chalcogenide transfer agent for improved aqueous quantum dot synthesis.

An innovative approach to chalcogenide precursor synthesis and their subsequent use for the production of CdX (X = S, Se, Te) quantum dots (QDs) in water under scalable and intensified continuous flow conditions is introduced. Herein, tris(2-carboxyethyl)phosphine (TCEP) is identified as a novel, efficient and water-soluble vehicle for chalcogenide transfer to form CdX QDs under aqueous conditions. A comprehensive exploration of critical process parameters, including pH, chalcogen excess, and residence time, utilizing a Design of Experiments (DoE) approach is reported. Reaction kinetics are investigated in real-time using a combination of in situ Raman spectroscopy and in-line 31P NMR spectroscopy. The conversion of TCEP into TCEP[double bond, length as m-dash]X (X = S, Se, Te) species is seamlessly adapted to continuous flow conditions. TCEP[double bond, length as m-dash]X precursors are subsequently employed in the synthesis of CdX QDs. Scalability trials are successfully demonstrated, with experiments conducted at flow rates of up to 80 mL min-1 using a commercially available mesofluidic flow reactor with favorable metrics. Furthermore, biocompatible and aqueous CdSe/ZnS core-shell QDs are for the first time prepared in flow within a fully concatenated process. These results emphasize the potential for widespread biological or industrial applications of this novel protocol.

Double resonant Raman scattering and photoluminescence mediated by heavy and light hole excitons in a (Cd, Mn)Te quantum well in external magnetic field

Double resonant Raman scattering and photoluminescence mediated by heavy and light hole excitons in a (Cd, Mn)Te quantum well in external magnetic field

Quantitative theory of backscattering in topological HgTe and (Hg,Mn)Te quantum wells: Acceptor states, Kondo effect, precessional dephasing, and bound magnetic polaron

We present the theory and numerical evaluations of the backscattering rate determined by acceptor holes or Mn spins in HgTe and (Hg,Mn)Te quantum wells in the quantum spin Hall regime. The role of anisotropic s-p and sp-d exchange interactions, Kondo coupling, Luttinger liquid effects, precessional dephasing, and bound magnetic polarons is quantified. The determined magnitude and temperature dependence of conductance are in accord with experimental results for HgTe and (Hg,Mn)Te quantum wells.

Effects of Charge Dopants in Quantum Spin Hall Materials.

Semiconductors' sensitivity to electrostatic gating and doping accounts for their widespread use in information communication and new energy technologies. It is demonstrated quantitatively and with no adjustable parameters that the presence of paramagnetic acceptor dopants elucidates a variety of hitherto puzzling properties of two-dimensional topological semiconductors at the topological phase transition and in the regime of the quantum spin Hall effect. The concepts of resonant states, charge correlation, Coulomb gap, exchange interaction between conducting electrons and holes localized on acceptors, strong coupling limit of the Kondo effect, and bound magnetic polaron explain a short topological protection length, high hole mobilities compared with electron mobilities, and different temperature dependence of the spin Hall resistance in HgTe and (Hg,Mn)Te quantum wells.

Heavy-metal-free blue-emitting ZnSe(Te) quantum dots: synthesis and light-emitting applications

Synthesis and improvement strategies of blue ZnSe(Te)-based QDs are reviewed and discussed. Recent advances regarding blue ZnSe(Te)-based light-emitting applications are systematically outlined, and existing challenges and prospects are provided.

Trion magnetic polarons in (Cd,Mn)Te/(Cd,Mn,Mg)Te quantum wells

A trion magnetic polaron formed by the exchange interaction of a positively charged exciton (trion) with localized spins of Mn$^{2+}$ ions is found experimentally in a 4\,nm wide Cd$_{0.98}$Mn$_{0.02}$Te/Cd$_{0.78}$Mn$_{0.02}$Mg$_{0.2}$Te quantum well containing resident holes. The experiment is performed at a temperature of 1.6 K using resonant excitation of the trion with circularly polarized light. The trion is formed from a resident hole, which is in a hole magnetic polaron state, and a photogenerated electron-hole pair. The dynamical evolution from the hole magnetic polaron to the trion magnetic polaron is accompanied by a spin-flip of the electron, which results in negative circular polarization of the photoluminescence. The degree of circular polarization reaches $-8\%$ at zero magnetic field and strongly decreases in transverse magnetic fields exceeding 0.2 T. Our model considerations show that different localization sizes of the resident and photogenerated holes and the resulting difference in their exchange interaction with the Mn$^{2+}$ spins maintains Mn spin polarization. The resulting exchange field of Mn acting on the electron provides a robust spin polarization of the trion magnetic polaron. We evaluate the electron exchange energy in the T$^+$MP to be 0.19~meV, and the T$^+$MP binding energy to be about 0.5 - 1 meV.

Energy and spin relaxation of excitons in CdMnTe/CdMgTe quantum wells

The energy and spin relaxation of excitons and trions is studied in the (Cd, Mn)Te/(Cd,Mg)Te quantum wells containing hole magnetic polarons by means of polarized photoluminescence technique. It was found that under circularly polarized photoexcitation with an energy higher than the ground state energy of the exciton, photoluminescence is also polarized. This fact points to the presence of the spin memory of the excitons in these systems. It is suggested that the long spin memory is due to the magnetic polaron formation stabilizing the spin of the exciton. Under quasiresonant excitation of the trion states an inverted sign of circular polarization degree is observed in the radiation spectrum. Such inversion of the polarization degree requires the special mechanism of the spin relaxation, associated with the spin flip of an electron in the trion magnetic polaron state. Application of the external magnetic field in the Voigt geometry leads to the depolarization of the excitonic radiation. The cause of this depolarization is the destabilization of the magnetic polaron and the shortening of the spin relaxation time of the exciton.

Magnetic ion relaxation time distribution within a quantum well

Time-resolved optically detected magnetic resonance (ODMR) is a valuable technique to study the local deformation of the crystal lattice around magnetic ion as well as the ion spin relaxation time. Here we utilize selective Mn-doping to additionally enhance the inherent locality of the ODMR technique. We present the time-resolved ODMR studies of single {(Cd,Mg)Te/(Cd,Mn)Te} quantum wells (QWs) with manganese ions located at different positions along the growth axis -- in the center or on the sides of the quantum well. We observe that spin-lattice relaxation of Mn$^{2+}$ significantly depends on the ion-carrier wavefunction overlap at low-magnetic fields. Interestingly, the effect is clearly observed in spite of very low carrier density, which suggests the potential for control of the Mn$^{2+}$ ion relaxation rate by means of the electric field in future experiments.

Investigation on nonlinear optical and optical limiting properties of Cd0.7Zn0.3Te quantum dots

Ternary alloyed Cd0.7Zn0.3Te colloidal quantum dots were synthesized under open atmosphere conditions. The Cd0.7Zn0.3Te quantum dots were characterized under XRD studies, DLS technique and optical absorption measurements. Using tauc and urbach relation the optical energy bandgap and urbach energy were obtained. The intensity depended nonlinear optical parameters of Cd0.7Zn0.3Te quantum dots were determined with Z-scan experimental setup using a green laser source working on continuous wave mode. The nonlinear absorption coefficient β, along with optical limiting capacity of Cd0.7Zn0.3Te quantum dot is investigated through the open aperture set up. The transmittance of Cd0.7Zn0.3Te quantum dot showed a sharp dip at the maximum intensity point of the laser which indicates reverse saturable absorption. The optical limiting threshold value for Cd0.7Zn0.3Te quantum dot was obtained around 4.6 KJ/cm2. The nonlinear refractive index of Cd0.7Zn0.3Te quantum dot is find out using closed aperture set up. The nonlinear refractive index with a negative sign was obtained for the Cd0.7Zn0.3Te quantum dot which is attributed to the self-defocusing phenomena. The nonlinear studies indicate that the sample exhibits good nonlinear properties and find applications in optical limiting materials.

Simulation of far-infrared HgTe/HgCdTe quantum-well vertical-cavity surface-emitting laser

We propose an original design for a far-infrared vertical-cavity surface-emitting laser based on a heterostructure with ten 4.7-nm HgTe / Hg0.3Cd0.7Te quantum wells grown on a [013] GaAs substrate. Its characteristics were calculated, and it was shown that the quality factor of the proposed resonator design can reach 248 at a wavelength of 23.25 μm, which is sufficient for the occurrence of laser generation at this wavelength at a temperature of 20 K and a concentration of nonequilibrium carriers of 1.4 × 1010 cm − 2. The estimate of the threshold pump intensity using CO2 laser radiation at a wavelength of 10.6 μm is 1 W / cm2, which makes it possible to expect continuous wave mode lasing. Such a low-threshold pump intensity should also lead to lasing at substantially higher temperatures.

Hot phonon effects and Auger recombination on 3 μm room temperature lasing in HgTe-based multiple quantum well diodes

We propose an electrically pumped laser diode based on multiple HgTe quantum wells with band structure engineered for Auger recombination suppression. A model for accounting for hot phonons is developed for calculating the nonequilibrium temperature of electrons and holes. Using a comprehensive model accounting for carrier drift and diffusion, Auger recombination, and hot-phonon effects, we predict of lasing at λ∼3 μm at room temperature in the 2.1 nm HgTe/Cd0.85Hg0.15Te quantum well heterostructure. The output power in the pulse can reach up to 600 mW for 100 nanosecond-duration pulses.

Excitonic fine structure of epitaxial Cd(Se,Te) on ZnTe type-II quantum dots

The structure of the ground-state exciton of Cd(Se,Te) quantum dots embedded in ZnTe matrix is studied experimentally using photoluminescence spectroscopy and theoretically using $\mathbf{k}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbf{p}$ and configuration interaction methods. The experiments reveal a considerable reduction of fine-structure splitting energy of the exciton with an increase of Se content in the dots. That effect is interpreted by theoretical calculations to originate due to the transition from spatially direct (type-I) to indirect (type-II) transition between electrons and holes in the dot induced by an increase of Se. The trends predicted by the theory match those of the experimental results very well. The theory identifies that the main mechanism causing elevated fine-structure energy, in particular in type-I dots, is due to the multipole expansion of the exchange interaction. Moreover, the theory reveals that for Se contents in the dot $>0.3$, there also exists a peculiar type of confinement showing signatures of both type I and type II and which exhibits extraordinary properties, such as an almost purely light hole character of exciton and toroidal shapes of hole states.

Bandgap engineering of ZnX (X = O, S, Se, Te) QDs/Graphene nanocomposites: Towards the designing of a highly efficient light-harvesting device

Increasing the power conversion efficiency (PCE) of photovoltaic devices (PVDs) is a topic of fundamental importance not only for academia but also for industry. In this study, we systematically investigated the electronic structure of ZnX (X = O, S, Se, and Te) quantum dots (QDs)/Graphene (G) nanohybrid systems by employing the density functional method. To understand the key issues, associated with photoinduced charge generation, generated charge separation, charge transfer process, recombination probability, and variations of PCE, we have considered different combinations of hybrid nanostructures composed with ZnX QD and G. The tuning of the electronic properties of hybrid systems has been done as a function of the size of ZnX QD's, chalcogenides, and hydrogen coverage percentage on G to design a highly efficient hybrid solar cell. We have also explored the photovoltaic efficiency of ZnX/ZnX’ (X/X' = O, S, Se, and Te) core/shell QDs/G to investigate the effect of core/shell QDs in composite nanomaterials. Bandgaps of ZnX QDs/G nanocomposites are very low (0.028eV–0.158 eV) which implies the probability of recombination of photo-induced separated charges at the heterojunction region is very high. After passivation with hydrogen atoms partially on the G, this unsavory circumstance is overcome. We have performed the bandgap and band alignment engineering of ZnX QDs/G hybrid nanostructures by varying the size of QD's and H-coverage percentage on G to form a type-II material which is very important to reduce the recombination rate and ultimately increase the PCE of the hybrid solar cells. Considering hydrogenated graphene (HG)-ZnX QDs nanocomposites we were achieving high PCE values (up to 30.25%), which implies our explored materials efficiency is exceptionally good and competitive with recently explored tandem/hybrid/latest perovskite solar cells.

Thiolate Etching Route for the Ripening of Uniform Ag2Te Quantum Dots Emitting in the Second Near-Infrared Window: Implication for Noninvasive In Vivo Imaging

Thiolate Etching Route for the Ripening of Uniform Ag₂Te Quantum Dots Emitting in the Second Near-Infrared Window: Implication for Noninvasive <i>In Vivo</i> Imaging

Angle-resolved optically detected magnetic resonance as a tool for strain determination in nanostructures

In this paper, we apply the angle-resolved optically detected magnetic resonance (ODMR) technique to study a series of strained (Cd, Mn)Te/(Cd, Mg)Te quantum wells (QWs) produced by molecular beam epitaxy. By analyzing characteristic features of ODMR angular scans, we determine the strain-induced axial-symmetry spin Hamiltonian parameter $D$ with neV precision. Furthermore, we use low-temperature optical reflectivity measurements and x-ray diffraction scans to evaluate the local strain present in the QW material. In our analysis, we take into account different thermal expansion coefficients of the GaAs substrate and CdTe buffer. The additional deformation due to the thermal expansion effects has the same magnitude as the deformation that originates from the different compositions of the samples. Based on the evaluated deformations and values of the strain-induced axial-symmetry spin Hamiltonian parameter $D$, we find the strain spin-lattice coefficient ${G}_{11}=(72.2\ifmmode\pm\else\textpm\fi{}1.9)$ neV for ${\mathrm{Mn}}^{2+}$ in CdTe and shear deformation potential $b=(\ensuremath{-}0.94\ifmmode\pm\else\textpm\fi{}0.11)$ eV for CdTe.

Water-Soluble High-Quality Ag2Te Quantum Dots Prepared by Mutual Adaptation of Synthesis and Surface Modification for In Vivo Imaging.

Near-infrared (NIR) in vivo fluorescence imaging has exhibited the distinct advantage of high optical resolution at deeper penetration into biological tissues. Ag2Te quantum dots (QDs), with a relatively narrow band gap, show great promise for fluorescence emission at long wavelengths in the second near-infrared (NIR-II) window for bioimaging. However, existing Ag2Te QDs have severely hindered the application of in vivo bioimaging due to their poor fluorescence brightness and stability, so it is important to prepare Ag2Te QDs with high quantum yield and stability as well as high biocompatibility in the NIR-II window. Herein, we designed an integrated method for the preparation of water-soluble Ag2Te QDs by mutual adaptation of QD synthesis and surface modification. We first synthesized high-quality Ag2Te QDs with different NIR-II emission wavelengths and the photoluminescence quantum yields (PLQYs) up to 6.51% by rapidly injecting the TBP-Te precursor into a hot solvent to form a highly fluorescent Ag2Te core. Then water-dispersible Ag2Te QDs were obtained by direct exchange of the hydrophobic Ag2Te QD surface ligands with thiol ligands. The PLQY of the water-soluble Ag2Te QDs obtained by this method can still be maintained at 4.94%. With these highly bright and stable Ag2Te QDs, the abdominal vessels, hindlimb arterial vessels, venous vessels, sacral lymph nodes, and tumor vessels were visualized non-invasively in vivo in the NIR-II window in mice. The results demonstrate that the integrated strategy of QD synthesis and modification provides valuable technical support for further in-depth applications of Ag2Te QDs.

Scalable production of intrinsic WX2 (X = S, Se, Te) quantum sheets for efficient hydrogen evolution electrocatalysis

Mass production of transition-metal dichalcogenides has attracted much attention to replace platinum-based catalysts for the hydrogen evolution reaction (HER). Herein, we demonstrate a general strategy for the scalable production of the intrinsic tungsten dichalcogenide (WX2 (X = S, Se, Te)) quantum sheets (QSs) by an all-physical top-down method. The method combines silica-assisted ball-milling and sonication-assisted solvent exfoliation and thus enables production of WS2 QSs, WSe2 QSs, and WTe2 QSs in exceedingly high yields of 28.2, 21.3, 19.9 wt%, respectively. The WX2 QSs are confirmed as intrinsic and defect-free, which could be determinative to their improved HER performance. The overpotentials of 285, 331, 435 mV at the current density of 10 mA cm−2 and Tafel slopes of 116, 78, 162 mV dec−1 in acidic media, as well as charge transfer resistance values of 171, 242, 1973 Ω, are derived for WS2 QSs, WSe2 QSs, and WTe2 QSs, respectively, which are much better than those of bulk materials. The WX2 QSs exhibit high stability during the electrocatalysis as well. This work offers a powerful approach for fabrication of intrinsic QSs as efficient and robust electrocatalysts.

Giant Thermoelectric Seebeck Coefficients in Tellurium Quantum Wires Formed Vertically in an Aluminum Oxide Layer by Electrical Breakdown.

High efficiency thermoelectric (TE) materials still require high thermopower for energy harvesting applications. A simple elemental metallic semiconductor, tellurium (Te), has been considered critical to realize highly efficient TE conversion due to having a large effective band valley degeneracy. This paper demonstrates a novel approach to directly probe the out-of-plane Seebeck coefficient for one-dimensional Te quantum wires (QWs) formed locally in the aluminum oxide layer by well-controlled electrical breakdown at 300 K. Surprisingly, the out-of-plane Seebeck coefficient for these Te QWs ≈ 0.8 mV/K at 300 K. This thermopower enhancement for Te QWs is due to Te intrinsic nested band structure and enhanced energy filtering at Te/AO interfaces. Theoretical calculations support the enhanced high Seebeck coefficient for elemental Te QWs in the oxide layer. The local-probed observation and detecting methodology used here offers a novel route to designing enhanced thermoelectric materials and devices in the future.

Au-Doped Ag2 Te Quantum Dots with Bright NIR-IIb Fluorescence for In Situ Monitoring of Angiogenesis and Arteriogenesis in a Hindlimb Ischemic Model.

Fluorescence located in 1500-1700nm (denoted as the near-infrared IIb window, NIR-IIb) has drawn great interest for bioimaging, owing to its ultrahigh tissue penetration depth and spatiotemporal resolution. Therefore, NIR-IIb fluorescent probes with high photoluminescence quantum yield (PLQY) and stability along with high biocompatibility are urgently pursued. Herein, a novel NIR-IIb fluorescent probe of Au-doped Ag2 Te (Au:Ag2 Te) quantum dots (QDs) is developed via a facile cation exchange method. The Au dopant concentration in the Ag2 Te QDs is tunable from 0% to 10% by controlling the ratio of supplied Au precursor to Ag2 Te QDs, resulting in a wide range of PL emission in the NIR-IIb window and a much-enhanced PL intensity. After surface modification, the Au:Ag2 Te QDs possess bright NIR-IIb emission, high colloidal stability and photostability, and decent biocompatibility. Further, in vivo monitoring of the process of angiogenesis and arteriogenesis in an ischemic hindlimb is successfully performed.

Optical manifestation of magnetic polarons bound to excitons and resident holes in a (Cd,Mn)Te quantum well

Photoluminescence and multiple ${\mathrm{Mn}}^{2+}$ spin-flip Raman scattering (SFRS) are studied in the (Cd,Mn)Te/(Cd,Mg)Te quantum well with excess resident hole concentration under resonance continuous-wave photoexcitation in the localized states of excitons and trions at $T=1.5$ K. A magnetic polaron shift of both exciton and trion photoluminescence is observed to be increasing with an increase in the magnetic field up to 6 T applied perpendicularly to the sample growth axis (Voigt geometry). Exciton magnetic polaron shift in magnetic field is attributed to the adiabatic energy transfer from the exchange reservoir to the Zeeman reservoir during the precession of the ${\mathrm{Mn}}^{2+}$ ions in the summary field of external magnetic and exchange hole fields. A mechanism of the magnetic polaron shift of trion photoluminescence is suggested. It is related to direct photoexcitation of the triplet trion state, which is stable in the conditions which are provided by the resident hole magnetic polarons. In these conditions, the ${\mathrm{Mn}}^{2+}$ magnetic moment experiences additional magnetization in the double exchange field provided by two holes in the triplet trion state. It is observed that the maximum efficiency of multiple ${\mathrm{Mn}}^{2+}$ spin-flip Raman scattering falls into the energy range corresponding to the states of localized excitons where the exciton magnetic polaron shift is observed. From this we conclude that the magnetic polaron state could be an intermediate state of the multiple ${\mathrm{Mn}}^{2+}$ SFRS process.