Triangular Quantum Dot Research Articles

Designing Two-Dimensional Versatile Room-Temperature Ferromagnets via Assembling Large-Scale Magnetic Quantum Dots.

Two-dimensional (2D) ferromagnets possess astonishing potential in new-concept spintronics. However, most of the reported intrinsic 2D ferromagnets show a low Curie temperature far below room temperature. Here, we propose a series of 2D magnetic covalent and metal organic frameworks (COFs/MOFs) by assembling triangular zigzag graphene quantum dots (TZGDs) with various linkages, involving small-sized TZGDs, nonmetal atoms, magnetic metal atoms, and molecules. Upon first-principles calculations, we demonstrate 2D magnetic semiconductors with an enhanced Curie temperature of up to 472 K can be realized through the strong p(d)-p direct exchange interaction between TZGDs and linkages. Particularly, the TZGD size hardly affects the Curie temperature, whereas linkages can modulate the Curie temperature significantly. The TZGD size and linkages can regulate the electronic and magnetic properties of TZGD-based 2D ferromagnets. Our results confirm the possibility of designing 2D ferromagnets based on TZGDs and motivate the research of 2D ferromagnets on magnetic quantum dots and molecular magnets.

The effect of shape and size in the stability of triangular Janus MoSSe quantum dots

Asymmetric Janus transition metal dichalcogenide MoSSe is a promising catalytic material due to the intrinsic in-plane dipole of its opposite faces. The atomic description of the structures observed by experimental techniques is relevant to tuning and optimizing its surface reaction processes. Furthermore, the experimentally observed triangular morphologies in MoSSe suggest that an analysis of the chemical environment of its edges is vital to understand its reactivity. Here we analyze the size-shape stability among different triangular structures-quantum- dots proposed from the ideal S(-1010) and Mo(10-10) terminations. Our stability analysis evidenced that the S–Se termination is more stable than Mo; moreover, as the size of the quantum dot increases, its stability increases as well. Besides, a trend is observed, with the appearance of elongated Mo-S/Se bonds at symmetric positions of the edges. Tersoff–Hamann scanning tunneling microscopy images for both faces of the stablest models are presented. Electrostatic potential isosurfaces denote that the basal plane on the S face of both configurations remains the region with more electron density concentration. These results point toward the differentiated activity over both faces. Finally, our study denotes the exact atomic arrangement on the edges of MoSSe quantum dots corresponding with the formation of S/Se dimers who decorates the edges and their role along with the faces as catalytic sites.

Edge effects on the intrinsic magnetism in ferromagnetic triangular phosphorene quantum dots using fully spin-polarized calculations

This project involves discovering the electronic and magnetic properties of nanometer-sized phosphorene structures with triangular shapes in both zigzag and armchair termination types. The goal is to discuss the relationship between the electronic states belonging to the different conditions of these phosphorene quantum dots and their intrinsic magnetic properties. For this purpose, we consider electronic interactions utilizing the spin-polarized density functional theory calculations, and then the results compare with the data generated from tight-binding calculations. Both descriptions yield mid-gap states in the spectrum of ferromagnetic structures. Our results in non-spin computations without any geometry optimization were matched by tight-binding calculations which shows that the tight-binding method is an inefficient approximation in analyzing the optimized spin samples. Unlike graphene, in our spin-polarized calculations, we have obtained empty mid-gap states in the spectrum of ferromagnetic triangular phosphorene quantum dots. The edge atoms of these structures are known as the magnetic atoms with an unequal contribution of spin up and spin down. To prevent deforming the initial structures, the dangling bonds at the edge atoms were passivated in two types, fully hydrogenated and partial passivation with oxygen atoms. Oxygen doping was required for introducing magnetism to the non-spin edges of the fully hydrogenated case.

The influence of AA and AB stacking on the ground state magnetic properties of triangular bilayer graphene quantum dots

Using the tight-binding model, we have studied the impact of AA and AB (Bernal) stacking on eigenstates and eigenvalues of bilayer graphene nanoflakes with triangular geometry. The zero eigenvalues of any single layer nanoflake will be doubled through AB stacking. However, we have noticed that in AA stacking, the eigenvalues are simply achievable through merging two sets of single layer eigenvalues that once added to and once subtracted from inter-layer hopping. Besides, in a single layer within some strict zero energies, the AB stacking still maintains them sublattice-polarized. However, the AA-stacking neglects the sublattice-polarized character of zero states in single layers constituent. Next, we consider onsite electron–electron interaction utilizing single-orbital Hubbard model to show how the different stacking affects the onset of the ground state magnetization in bilayer quantum dots with different stackings. Especially, in AA stacking bilayer, the system always needs a nonzero onsite interaction strength (Uc) to be magnetized.

Spin filtering and magnetically-controlled quantum switch in multiple triangular rings consisting of quantum dots

Electron transport characteristics through a system consisting of a series of triangular quantum dot rings are studied utilizing the non-equilibrium Green’s function. In the presence of a magnetic field, an additional anti-resonance point occurs in the conductance spectrum. As the number of triangular quantum dot rings increases, two anti-resonance points evolve into two well-defined insulating bands. The insulating band disappears as the magnetic flux takes an appropriate value, and therefore, an effective magnetically-controlled quantum switch can be achieved. If a Zeeman magnetic field is introduced, a spin-polarized window (SPW) can be formed, suggesting a physical scheme of a spin filtering. As the intradot Coulomb interactions are taken into consideration, more SPWs can be obtained. This work sheds lights onto the design of future quantum devices.

The effect of size and applied electric field on the spin switch in a two-electron graphene quantum dot and graphene qubit

Abstract To get a carbon-based qubit, we pay attention to a triangular graphene quantum dot with two electrons occupying the degenerate shell. The spin of the dot is determined by the energy difference between the lowest spin-singlet and spin-triplet states. The energy difference is analyzed as a function of the dot size and an applied electric field. For different sizes, the curves of the energy difference share a similar outline: a brief oscillation as the initial party and a single big peak as the main party. Based on the obtained results, we analyze the spin switches controlled by the weak electric field, as well as by the strong electric field. The weak and strong electric fields have their own advantages in controlling spin switches. When the weak electric field is used for controlling spin switches, the graphene quantum dot with the larger size is available as a qubit. Our findings can be helpful for using a graphene quantum dot as a qubit.

Comparison of bound magneto-polaron in circular, elliptical, and triangular quantum dot qubit

In the present work, we consider an electron which is strongly coupled to the LO-phonon in circular, elliptical, and triangular quantum dots with Coulomb impurity. The eigenenergies and eigenfunctions of the ground and the first-excited states of the electron are obtained under magnetic and electric fields by using the Pekar variational method. We have also obtained the Shannon entropy and the electron probability density. This system can be applied as a two-level qubit. The entropy shows the oscillatory periodic evolution as function of the time due to the form of the confinement. It is seen that entropy has an arrangement periodic behavior for the higher symmetry (circular quantum dot).

Aharonov–Bohm oscillations and equilibrium current distributions in open quantum dot and in ring interferometer

Magnetotransport in two submicron-sized devices formed on the basis of GaAs/AlGaAs structures has been simulated using nonequilibrium Green functions. The effect of a perpendicular magnetic field on quantum transport in a quasi-one-dimensional quantum dot and in an Aharonov–Bohm interferometer has been analyzed in a single-particle approximation. Magnetic field oscillations of two-terminal conductance of the devices, equilibrium (persistent) current distributions and magnetic moment generated in the devices by persistent currents have been determined using numerical methods. Correlations between the magnetic moment, magnetic field oscillations of conductance and energy resonance in a specific magnetic field have been traced. Sufficiently regular conductance oscillations similar to Aharonov–Bohm ones have been revealed for a quasi-one-dimensional quantum dot at small magnetic fields (0.05–0.4 T). For a ring interferometer the contribution to the total equilibrium current and magnetic moment at a specific energy may change abruptly both in magnitude and in sign as a result of changing magnetic field within one Aharonov–Bohm oscillation. We show that the conductance of an interferometer is determined not by the number of modes propagating in the ring but rather by the effect of triangular quantum dots at the ring entrance that produce a strong reflection. The period of the calculated Aharonov–Bohm oscillations is in agreement with the measurement results for these devices.

Erratum to: Highly efficient and stable white LEDs based on pure red narrow bandwidth emission triangular carbon quantum dots for wide-color gamut backlight displays

Erratum to: Highly efficient and stable white LEDs based on pure red narrow bandwidth emission triangular carbon quantum dots for wide-color gamut backlight displays

Aharonov—Bohm oscillations and distributions of equilibrium current in open quantum dot and ring interferometer

Magnetotransport in submicron devices formed on the basis of GaAs/AlGaAs structures is simulated by the method of nonequilibrium Green functions. In the one-particle approximation, the influence of a perpendicular magnetic field on electron transmission through a quasi-one-dimensional quantum dot and the Aharonov—Bohm interferometer is considered. Two-terminal conductance and magnetic moment of the devices are calculated. Two-dimensional patterns of equilibrium (persistent) currents are obtained. The correlations between energy dependences of magnetic moment and conductance are considered. For the quasi-one-dimensional quantum dot, regular conductance oscillations similar to the ABOs were found at low magnetic fields (0.05—0.4 T). In the case of a ring interferometer, the contribution to the total equilibrium current and magnetic moment at a given energy can change sharply both in magnitude and in sign when the magnetic field changes within the same Aharonov—Bohm oscillation. The conductance through the interferometer is determined not by the number of propagating modes, but rather by the influence of triangular quantum dots at the entrances to the ring, causing back scattering. Period of calculated ABOs corresponds to that measured for these devices.

ZnCl2 Enabled Synthesis of Highly Crystalline and Emissive Carbon Dots with Exceptional Capability to Generate O2⋅–

Summary Highly crystalline and emissive carbon dots (CDs) are firstly synthesized via an ionothermal method in organic solvents. In contrast to well-established approaches, ZnCl2 is utilized as the pyrolysis-promoting agent to prepare emissive CDs with tunable wavelengths, directly by carbonization from different O and N precursors under a mild reaction condition at 210°C. More than 50% synthetic yield and 39% photoluminescent quantum yield (blue CD) are obtained. XAFS measurements, in conjunction with TEM, HAADF-STEM, UPS, and ESR analyses, allow us to obtain energy diagram configuration and confirm the luminescent nature of these hard-core structured CDs. Importantly, the resulting CDs exhibit excellent capability to photogenerate superoxide radical anion (O2⋅–), which can be exemplified by both photodynamic therapy and photooxidative cyclization synthesis of tetrahydroquinolines. Representative pharmaceutical mifepristone can be derivatized in 90% yield within 10 min of irradiation of the CDs in an elegant flow reactor.

Dynamic manipulation of probe pulse and coherent generation of beating signals based on tunneling-induced inference in triangular quantum dot molecules**Project supported by the National Natural Science Foundation of China (Grant Nos. 11704151 and 11247201), the Twelfth Five-Year Program for Science and Technology of Education Department of Jilin Province, China (Grant No. 20150215), and the Innovation and Entrepreneurship Program of Liaoning

We investigate the dynamic propagation of a probe field via the tunneling-induced interference effect in a triple model of quantum dot molecules. By theoretical analysis and numerical simulation, we find that the number of transparency window relate to the energy splitting and the group velocity of probe field can be effectively controlled by the tunneling coupling intensity. In addition, in the process of light storage and retrieval, when the excited states have no energy splitting in the storage stage but opposite values of the energy splitting in the retrieval stage, the beating signals can be generated.

Electrical spin switch in a two-electron triangular graphene quantum dot

Abstract We study a triangular graphene quantum dot with two electrons filled in the degenerate shell. The electronic structures of the dot are calculated as a function of an electric field using a combination of tight-binding, Hartree–Fock and exact-diagonalization methods. To use the total spin ( S ) of the dot as a qubit, we pay attention to the energy difference between the lowest S = 0 and S = 1 states. The energy difference increases unexpectedly up to a rather large value before a crossing between the lowest S = 0 and S = 1 states occurs. To understand the unexpected change, we propose an approach to analyze the total interaction energy of the two-electron states. As the electric field changes, the total interaction energies of the lowest S = 0 and S = 1 states exhibit three characteristics in turn: oscillation, transition and stability. Our findings support the possibility to use a triangular graphene quantum dot as a qubit of a carbon-based quantum computer.

Highly efficient and stable white LEDs based on pure red narrow bandwidth emission triangular carbon quantum dots for wide-color gamut backlight displays

High-performance white light-emitting diodes (WLEDs) hold great potential for the next-generation backlight display applications. However, achieving highly efficient and stable WLEDs with wide-color-gamut has remained a formidable goal. Reported here is the first example of pure red narrow bandwidth emission triangular CQDs (PR-NBE-T-CQDs) with photoluminescence peaking at 610 nm. The PR-NBE-T-CQDs synthesized from resorcinol show high quantum yield (QY) of 72% with small full width at half maximum of 33 nm. By simply controlling the reaction time, pure green (PG-) NBE-T-CQDs with high QY of 75% were also obtained. Highly efficient and stable WLEDs with wide-color-gamut based on PR- and PG-NBE-T-CQDs was achieved. This WLED showed a remarkable wide-color gamut of 110% NTSC and high power efficiency of 86.5 lumens per Watt. Furthermore, such WLEDs demonstrate outstanding stability. This work will set the stage for developing highly efficient, low cost and environment-friendly WLEDs based on CQDs for the next-generation wide-color gamut backlight displays.

Electronic properties and optical response of triangular and hexagonal MoS2 quantum dots. A DFT approach

The energy states of triangular and hexagonal MoS2 quantum dots are studies with the use of density functional theory, varying the dot size. The system edges are assumed to be passivated with sulfur-hydrogen atoms. In each case, spin-up and spin-down polarizations are investigated via the calculation of the energy gaps, density of states and the interband optical response. The structures are found to be small gap semiconductors. In addition, from the calculated real and imaginary parts of the dielectric function the static index of refraction and the so-called energy loss are evaluated. The effect of the particular dot geometry on the physical quantities under study is specially discussed. It is found that the specific triangular configuration with a total of 42 atoms in the border exhibits a very small energy bandgap associated with the spin-up polarization, which leads to a significant deviation of the value of the related static index of refraction, compared with the remaining structures investigated. From the first-principles calculation it has been also possible to evaluate the spin-polarization and estimate the total magnetic moment, which ranges from ∼2μB to ~14μB, depending on the dot size and geometry.

Electric-field control of the degenerate shell in a triangular graphene quantum dot

Abstract For a triangular graphene quantum dot with up to two electrons filled in the degenerate shell, we calculate the eigenstates as functions of an electric field, using the tight-binding method, the Hartree-Fock method and the exact-diagonalization method. When the electric field is weaker, the eigenstates of the degenerate shell mix with each other. As the electric field becomes stronger, the eigenstates of the degenerate shell tend to be stable. We focus on the evolution of both the ground levels and the total spins in the electric field. Based on analyzing the chemical potentials, we present the control scheme of the shell filling and the total spin by the electric field. The results support the possibility to use the system as a spin memory device or an electron-transport switch.

Ultrastable and Low‐Threshold Random Lasing from Narrow‐Bandwidth‐Emission Triangular Carbon Quantum Dots

AbstractThe development of ultrastable and low‐threshold random lasers has long been the subject of intense academic research, in anticipation for widespread applications spanning from military weapon devices, biomedical therapy, and materials processing to energy savings. Reported here is the first example of ultrastable and low‐threshold random lasing emissions ranging from blue to red based on narrow bandwidth emission triangular carbon quantum dots (CQDs). The lasing thresholds of the blue, green, and red random lasers are determined to be as low as 0.087, 0.052, and 0.048 mJ cm−2, and the corresponding narrow full widths at half maximum are 0.9, 0.37, and 0.82 nm, respectively. Furthermore, such random lasers demonstrate outstanding stability. This work will inspire further research on lasers operating in other modes, based on the CQDs, such as the whispering‐gallery mode and the Fabry–Pérot mode, which will expedite the development of the next‐generation of ultrastable and low‐threshold CQDs‐based laser technology.

Phase diagram characterized by transmission in a triangular quantum dot**Project supported by the National Natural Science Foundation of China (Grant Nos. 11174228 and 10874132).

We propose a theoretical model to detect the quantum phase transition in a triangular quantum dot molecule with frustration. The boundaries of the phase diagram are accurately determined by the transmission. For small frustration t, as the interdot Coulomb repulsion V increases, the system undergoes a Kosterlitz–Thouless (KT) transition from the Kondo resonance state with a transmission peak at zero energy to the Coulomb blocked state with zero transmission, which is followed by a first transition to the V-induced resonance (VIR) state with unitary transmission. For large frustration t, as V increases, the orbital spin singlet without transmission transits to the VIR state through a KT transition.

Tunneling-induced ultraslow solitons in triangular quantum dot molecules

We theoretically investigate the propagation of a weak probe laser pulse in a triangular quantum dot molecules scheme based on the tunneling induced transparency. We find that the ultraslow optical solitons can be realized due to the destructive quantum interference induced by the interdot tunneling coupling which can be adjusted by the gate voltage appropriately. This work may provide practical applications such as electro-optic modulated devices and other information processes in semiconductor quantum dots structure.

Multilayer phosphorene quantum dots in an electric field: Energy levels and optical absorption

Triangular and hexagonal multilayer phosphorene quantum dots with armchair and zigzag terminations are investigated with the orthogonal tight-binding model. The effect of increasing the number of layers is revealed. The obtained results show that in a small size multilayer quantum dot, the edge states are as sensitive to the out-of-plane external electric fields as the edge states in a single layer dot to the in-plane external electric fields. The investigated optical absorption cross sections show that armchair phosphorene quantum dots have a regular behavior which should be useful for infrared detectors. In particular, it was found that in hexagonal armchair phosphorene dots, absorption peaks can be increased, decreased, or totally removed from the low-energy region depending on the orientation of the applied electric field. The effect of spurious doping can suppress the transitions <0.4 eV, while the effect of the finite temperature is almost negligible.