Triangular Quantum Dot Research Articles

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.

Stability and electronic properties of edge functionalized silicene quantum dots: A first principles study

The stability and electronic properties of hexagonal and triangular silicene quantum dots are investigated under the effect of edge passivation by different elements and molecular groups. The structures experience a considerable alternation in shape depending on the attached elements or groups. The most noticeable alternations occur in zigzag triangular flakes passivated with sulfur and in all the selected flakes when OH groups are attached to their edge atoms. The resulting structure has a spherical shape with a large total dipole moment. All the studied clusters have been proven to be stable by the calculated positive binding energies. A flexible structure transformation from insulator (conductor) to conductor (insulator) is obtained in zigzag hexagonal-H (zigzag triangular-H) and zigzag hexagonal-S (zigzag triangular-OH), respectively. The magnetic properties of the triangular zigzag depend on the parity of the total number of Si atoms such that flakes with an even number of Si atoms will have antiferromagnetic properties while flakes with an odd number of Si atoms can have ferromagnetic or antiferromagnetic properties depending on the attached element or group. Thus, a proper choice of the attached functional groups or elements to silicene flakes allows tailoring of their properties to different application. In particular, hydrogenated or fluorinated flakes are highly interactive with the surrounding and can be used for sensor applications while clusters passivated with S or OH are insensitive to edge defects and have tunable electronic properties that make them promising in semiconductor device applications.

Edge magnetism in triangular silicene quantum dots

The interplay among electron-electron and intrinsic spin-orbit interactions (ISOI) at the zigzag edge of triangular quantum dots (ZTQDs) was analyzed by means of the mean-field Kane-Mele-Hubbard model. It was shown that the magnetic anisotropy (in-plane and out-of-plane magnetic configurations) significantly modifies electronic and magnetic properties of ZTQDs. The in-plane configuration is more energetically stable than the other one. However, its magnetization becomes easier depolarized as a continuous function as strengths of electric field E and ISOI increase. This is in contrast to the out-of-plane magnetization, which is more robust and becomes depolarized in a step-like way. Moreover, the energy gap only vanishes in the out-of-plane case by adjusting E and ISOI strengths, and thus we cannot expect a topological insulator phase in the in-plane magnetic configuration.

Controllable synthesis of biocompatible triangular quantum dots with near-infrared emitting and their application for in vivo lymph node imaging

Controllable synthesis of biocompatible triangular quantum dots with near-infrared emitting and their application for in vivo lymph node imaging

Engineering triangular carbon quantum dots with unprecedented narrow bandwidth emission for multicolored LEDs

Carbon quantum dots (CQDs) have emerged as promising materials for optoelectronic applications on account of carbon’s intrinsic merits of high stability, low cost, and environment-friendliness. However, the CQDs usually give broad emission with full width at half maximum exceeding 80 nm, which fundamentally limit their display applications. Here we demonstrate multicolored narrow bandwidth emission (full width at half maximum of 30 nm) from triangular CQDs with a quantum yield up to 54–72%. Detailed structural and optical characterizations together with theoretical calculations reveal that the molecular purity and crystalline perfection of the triangular CQDs are key to the high color-purity. Moreover, multicolored light-emitting diodes based on these CQDs display good stability, high color-purity, and high-performance with maximum luminance of 1882–4762 cd m−2 and current efficiency of 1.22–5.11 cd A−1. This work will set the stage for developing next-generation high-performance CQDs-based light-emitting diodes.

Suppression of spin and optical gaps in phosphorene quantum dots

Electronic structure and optical properties of triangular phosphorene quantum dots have been investigated theoretically. Based on systematic configuration interaction calculations, the ground and excited states of the interacting many-electron system together with its optical absorption spectrum are obtained. For the nanodot with 60 phosphorus atoms in various dielectric environments, it is found that the spin gap of the correlated system surprisingly overlaps its optical gap over a large range of the effective dielectric constant. The overlapping of the spin and optical gaps can be attributed to the fact that the extra correlation energy in the spin singlet almost compensates the exchange energy in the spin triplet in the presence of strong long-range electron-electron interactions. Moreover, both the spin and optical gaps are shown to be greatly suppressed as the screening effect becomes strong. When the dielectric constant decreases below 2.65, it is seen that the spin gap becomes negative and the quantum dot undergoes a phase transition from nonmagnetic to ferromagnetic. Our results are compared with the previous experimental and theoretical works.

Electronic properties and optical absorption of a phosphorene quantum dot

Using the tight-binding Hamiltonian approach, we theoretically study the electronic and optical properties of a triangular phosphorene quantum dot (PQD) including one normal zigzag edge and two skewed armchair edges (ZAA-PQD). It is shown that the energy spectrum can be classified into the filled band (FB), the zero-energy band (ZB), and the unfilled band (UB). Numerical calculations of the FB, ZB, and UB probability distributions show that the FB and the UB correspond to the bulk states, while the ZB corresponds to the edge states, which appear on all of the three edges of the ZAA-PQD sharply different from the other PQDs. We also find that the strains and the electric fields can affect the energy levels inhomogeneously. Then the optical properties of the ZAA-PQD are investigated. There appear some strong low-energy optical absorption peaks indicating its sensitive low-energy optical response that is absent in other PQDs. Moreover, the strains and the electric fields can make inhomogeneous influences on the optical spectrum of the ZAA-PQD. This work may provide a useful reference for designing the electrical, mechanical, and optical PQD devices.

Electrically-induced polarization selection rules of a graphene quantum dot

We study theoretically the single-electron triangular zigzag graphene quantum dot in uniform in-plane electric fields. The absorption spectra of the dot are calculated by the tight-binding method. The energy spectra and the distribution of wave functions are also presented to analyse the absorption spectra. The orthogonal zero-energy eigenstates are arranged along to the direction of the external field. The remarkable result is that all intraband transitions and some interband transitions are forbidden when the absorbed light is polarized along the direction of the electric field. With x-direction electric field, all intraband absorption is y polarized due to the electric-field-direction-polarization selection rule. Moreover, with y-direction electric field, all absorption is either x or y polarized due to the parity selection rule as well as to the electric-field-direction-polarization selection rule. Our calculation shows that the formation of the absorption spectra is co-decided by the polarization selection rules and the overlap between the eigenstates of the transition.

Tuning electronic properties in graphene quantum dots by chemical functionalization: Density functional theory calculations

The energy gap and dipole moment of chemically functionalized graphene quantum dots are investigated by density functional theory. The energy gap can be tuned through edge passivation by different elements or groups. Edge passivation by oxygen considerably decreases the energy gap in hexagonal nanodots. Edge states in triangular quantum dots can also be manipulated by passivation with fluorine. The dipole moment depends on: (a) shape and edge termination of the quantum dot, (b) attached group, and (c) position to which the groups are attached. Depending on the position of attached groups, the total dipole can be increased, decreased, or eliminated.

First principles study of edge carboxylated graphene quantum dots

The structure stability and electronic properties of edge carboxylated hexagonal and triangular graphene quantum dots are investigated using density functional theory. The calculated binding energies show that the hexagonal clusters with armchair edges have the highest stability among all the quantum dots. The binding energy of carboxylated graphene quantum dots increases by increasing the number of carboxyl groups. Our study shows that the total dipole moment significantly increases by adding COOH with the highest value observed in triangular clusters. The edge states in triangular graphene quantum dots with zigzag edges produce completely different energy spectrum from other dots: (a) the energy gap in triangular zigzag is very small as compared to other clusters and (b) the highest occupied molecular orbital is localized at the edges which is in contrast to other clusters where it is distributed over the cluster surface. The enhanced reactivity and the controllable energy gap by shape and edge termination make graphene quantum dots ideal for various nanodevice applications such as sensors. The infrared spectra are presented to confirm the stability of the quantum dots.

Temperature effects on bound polaron in triangular quantum dot qubit subjected to an electromagnetic field

In this work, the variational method of the Pekar type is used to theoretically study the temperature effects on the bound polaron in the triangular quantum dot (QD) qubit subjected to an electromagnetic field. We analyze and discuss in detail the numerical results and show that the relationships of the ground and first excited state energies, the electron probability density and the electron oscillation period in the superposition state of the ground state and the first-excited state with the temperature, the cyclotron frequency, the electron-phonon coupling constant, the electric field strength, the confinement strength and the Coulomb impurity potential, respectively.

Shape effect on the second order nonlinear optical properties in triangular quantum dots with applied electric field

The shape effect of triangular quantum dots (TQDs) on the second-harmonic generations (SHG) coefficient and the optical rectification (OR) coefficient with the applied electric field has been theoretically investigated, by using the compact-density-matrix approach and iterative method. Within the effective-mass approximation, the energy levels and the wave functions of the system are obtained. The results show that both the strength of external electric field and the shape of triangular quantum dots have a significant influence on the second-harmonic generations and the optical rectification.

Energy spectrums of bilayer triangular phosphorene quantum dots and antidots

We theoretically investigate the confined states of the bilayer triangular phosphorene dots and antidots by means of the tight-binding approach. The dependence of the energy levels on the size, the type of the boundary edges, and the orientation of the dots and antidots, and the influences of the electric and magnetic fields on the energy levels, are all completely analyzed. It is found that the energy level numbers of the bilayer dots and antidots are determined by the energy levels in two layers. The external electric field can effectively tune the energy levels of the edge states in both layers to move in opposite directions. With the increase of the magnetic field, the magnetic energy levels can approach the Landau levels of the phosphorene monolayer, the phosphorene bilayer, or both, depending on the specific geometry of the monolayer-bilayer hybrid phosphorene quantum dots. This research should be helpful for the overall understanding of the electronic properties of the multilayer hybrid phosphorene nanostructures and designing the corresponding phosphorene devices.

Fano stability diagram of a symmetric triple quantum dot

The Fano factor stability diagram of a C$_{3v}$ symmetric triangular quantum dot is analysed for increasing electron fillings $N$. At low filling, conventional Poissonian and sub-Poissonian behavior is found. At larger filling, $N\ge 2$, a breaking of the electron-hole symmetry is manifested in super-Poissonian noise with a peculiar bias voltage dependence of the Fano factor at Coulomb and interference blockade. An analysis of the Fano map unravels a nontrivial electron bunching mechanism arising from the presence of degenerate many-body states combined with orbital interference and Coulomb interactions. An expression for the associated dark states is provided for generic $N$.

Screened spin−1 and −1/2 Kondo effect in a triangular quantum dot system with interdot Coulomb repulsion

By means of the numerical renormalization group (NRG) technique, we study the low temperature transport property and the phase transition for a triangular triple quantum dot system, including two centered dots (dot 1 and 2) and one side dot (dot 3). We focus on the effect of interdot repulsion V between two centered dots in a wide range of the interdot hopping tij (i,j = 1,2,3). When the hoppings between the centered dot and the side dot are symmetric, i.e., t13 = t23, and that between two centered dots t12 is small, two centered dots form a spin triplet when V is absent, and a totally screened spin−1 Kondo effect is observed. In this case, one has a spin 1 that is partially screened by the leads as in the usual spin−1 Kondo model, and the remaining spin 1/2 degree of freedom forms a singlet with the side dot. As V is large enough, one of the centered dots is singly occupied, while the other one is empty. The spin−1/2 Kondo effect is found when t13 is small. For large t12, two centered dots form a spin singlet when V = 0, leading to zero conductance. As V is large enough, the spin−1/2 Kondo effect is recovered in the case of small t13. For asymmetric t13≠t23 and small t12, a crossover is found as V increases in comparison with a first order quantum phase transition for the symmetric case. In the regime of large V, the spin−1/2 Kondo effect could also be found when both t13 and t23 are small. We demonstrate the present model is similar to the side-coupled double dot system in some appropriate regimes, and it appears as a possible realization of side-controllable molecular electronics and spintronics devices.

Decoherence time, hydrogenic-like impurity effect and Shannon entropy on polaron in RbCl triangular quantum dot qubit

Using Pekar variational method, Eigen energies of the ground and first excited states of the polaron in triangular bound and Coulomb potential quantum dot are derived in view of investigating the density of probability, the decoherence time and the Shannon entropy. Numerical analysis show that the decoherence time is decreasing function of polaron radius and the strength of the Coulombic impurity and the increase function of dispersion coefficient. These results suggest that the decrease of polaron radius and Coulombic impurity lead to the increase of coherence time. Also the entropy shows the oscillatory periodic evolution as function of the time due to the triangular form of the confinement. It’s also seen that entropy is periodic for the lower value of Coulomb impurity parameter and for the higher value of the polaronic radius.

The optical selection rules of a graphene quantum dot in external electric fields

We study theoretically the triangular zigzag graphene quantum dot in electric fields and find that there exist the remarkable optical selection rules in the FIR spectra.

A comparative study on the edge states in phosphorene quantum dots and rings

Using the tight-binding Hamiltonian approach, we comparatively investigate the energy spectrums of triangular zigzag phosphorene quantum dots (PQDs) and rings (PQRs), as well as their potential applications. In comparison with the outer edge states in the PQD, new extra inner edge states can be produced in the PQR by its internal hole. A transition from the uncoupled to coupled edge states can be induced by decreasing the width between the outer and inner edges of the PQR. Also, the edge states in PQD/PQR are all anisotropically localized in one side, rather than three sides as in triangular graphene quantum dots (QDs) and rings (QRs). Furthermore, the PQD/PQR energy levels can be anisotropically manipulated by the external electric fields and strains, clearly demonstrating their potential applications in field effect transistors or electromechanical devices. In the meanwhile, we also consider the electron probability distributions corresponding to the different energy levels, clearly exposing the characteristics of the PQD/PQR energy levels. The comparison between the asymmetrical triangular PQDs/PQRs and the symmetrical triangular QDs/QRs in graphene should be instructive for understanding the similar triangular QDs/QRs in other two-dimensional layered materials, as well as other types of QDs/QRs of different shapes.

Application of non-extensive entropy to study of decoherence of RbCl quantum dot qubit: Tsallis entropy

In this work, an electron which is strongly coupled to the LO-phonon in triangular quantum dots with Coulomb impurity is considered. The eigenenergies and eigenfunctions of the ground and the first-excited states of the electron are obtained using the Pekar variational method. We have studied decoherence of RbCl quantum dot qubit using the non-extensive entropy (Tsallis entropy) for different values of Coulomb impurity parameter, polaronic radius and electron-LO phonon coupling strength. Numerical analysis shows that the entropy has the oscillatory periodic evolution as function of the time due to the triangular form of the confinement. It is found that entropy oscillates under a standing wave envelope with increasing the Coulomb impurity parameter, electron-LO phonon coupling strength and polaronic radius. With reducing the non-extensive parameter q, the entropy increases and thereby we can miss information about the system.

The effect of temperature on the lifetime and Shannon entropy of a magnetopolaron in a RbCl triangular quantum dot

Using the Pekar variational method, the ground and first excited state energy of the magnetopolaron in a RbCl triangular quantum dot are derived. This system in a quantum dot is treated as a two-level quantum dot system. The density of probability and the Shannon entropy are derived and the influence of the magnetic field on the Shannon entropy is studied numerically. Applying the Fermi golden rule, the magnetopolaron lifetime is derived. The magnetopolaron lifetime increases with an increase of the cyclotron frequency and the polaron radius and decreases with temperature. This magnetopolaron lifetime is a periodic oscillatory function of the polar angle. These results may be very helpful for the transmission of information in nanostructures.